[Bug driver/70936] Hard-coded C++ header paths and relocation problem on Windows

["lh_mouse at 126 dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70936

--- Comment #6 from lh_mouse <lh_mouse at 126 dot com> ---
(In reply to Jonathan Wakely from comment #4)
> Please provide the missing information that https://gcc.gnu.org/bugs/ asks
> for, so we know how you configured GCC.

The following command shows how gcc has been configured:
```
E:\>i686-w64-mingw32-gcc -v
Using built-in specs.
COLLECT_GCC=i686-w64-mingw32-gcc
COLLECT_LTO_WRAPPER=C:/MinGW/MSYS2/mingw32/lib/gcc/i686-w64-mingw32/6.1.1/lto-wrapper.exe
Target: i686-w64-mingw32
Configured with: ../gcc/configure --prefix=/mingw32
--with-local-prefix=/mingw32/local --build=i686-w64-mingw32
--host=i686-w64-mingw32 --target=i686-w64-mingw32
--with-native-system-header-dir=/mingw
32/i686-w64-mingw32/include --libexecdir=/mingw32/lib
--with-gxx-include-dir=/mingw32/include/c++/6.1.1 --enable-bootstrap
--with-arch=i686 --with-tune=generic --enable-languages=c,lto,c++ --enable-sh
ared --enable-static --enable-libatomic --enable-threads=mcf --enable-graphite
--enable-fully-dynamic-string --enable-libstdcxx-time=yes
--disable-libstdcxx-pch --disable-libstdcxx-debug --enable-vers
ion-specific-runtime-libs --disable-isl-version-check --enable-lto
--enable-libgomp --disable-multilib --enable-checking=release --disable-rpath
--disable-win32-registry --disable-nls --disable-werror
 --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw32
--with-mpfr=/mingw32 --with-mpc=/mingw32 --with-isl=/mingw32
--with-pkgversion='gcc-6-branch HEAD with MCF thread model, built
 by LH_Mouse.' --with-bugurl=http://cpp.ra2diy.com/ --with-gnu-as --with-gnu-ld
--disable-tls --disable-sjlj-exceptions --with-dwarf2
Thread model: mcf
gcc version 6.1.1 20160511 (gcc-6-branch HEAD with MCF thread model, built by
LH_Mouse.)
```



A minimal testcase with a relocated toolchain is:
```
LH_Mouse@LH-PC  /mingw32/bin
$ echo '#include <cstdlib>' | g++ -v -x c++ -
Using built-in specs.
COLLECT_GCC=C:\new_directory\MSYS2\mingw32\bin\g++.exe
COLLECT_LTO_WRAPPER=C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/lto-wrapper.exe
Target: i686-w64-mingw32
Configured with: ../gcc/configure --prefix=/mingw32
--with-local-prefix=/mingw32/local --build=i686-w64-mingw32
--host=i686-w64-mingw32 --target=i686-w64-mingw32
--with-native-system-header-dir=/mingw
32/i686-w64-mingw32/include --libexecdir=/mingw32/lib
--with-gxx-include-dir=/mingw32/include/c++/6.1.1 --enable-bootstrap
--with-arch=i686 --with-tune=generic --enable-languages=c,lto,c++ --enable-sh
ared --enable-static --enable-libatomic --enable-threads=mcf --enable-graphite
--enable-fully-dynamic-string --enable-libstdcxx-time=yes
--disable-libstdcxx-pch --disable-libstdcxx-debug --enable-vers
ion-specific-runtime-libs --disable-isl-version-check --enable-lto
--enable-libgomp --disable-multilib --enable-checking=release --disable-rpath
--disable-win32-registry --disable-nls --disable-werror
 --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw32
--with-mpfr=/mingw32 --with-mpc=/mingw32 --with-isl=/mingw32
--with-pkgversion='gcc-6-branch HEAD with MCF thread model, built
 by LH_Mouse.' --with-bugurl=http://cpp.ra2diy.com/ --with-gnu-as --with-gnu-ld
--disable-tls --disable-sjlj-exceptions --with-dwarf2
Thread model: mcf
gcc version 6.1.1 20160511 (gcc-6-branch HEAD with MCF thread model, built by
LH_Mouse.)
COLLECT_GCC_OPTIONS='-v' '-shared-libgcc' '-mtune=generic' '-march=i686'

C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/cc1plus.exe
-quiet -v -iprefix
C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/
-U_REENTRANT - -quiet -dumpba
se - -mtune=generic -march=i686 -auxbase - -version -o
C:\Users\LH_Mouse\AppData\Local\Temp\ccUWITfc.s
GNU C++14 (gcc-6-branch HEAD with MCF thread model, built by LH_Mouse.) version
6.1.1 20160511 (i686-w64-mingw32)
        compiled by GNU C version 6.1.1 20160511, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring duplicate directory
"C:/new_directory/MSYS2/mingw32/lib/gcc/../../lib/gcc/i686-w64-mingw32/6.1.1/include"
ignoring nonexistent directory "C:/MinGW/MSYS2/mingw32/include"
ignoring nonexistent directory "/mingw32/include"
ignoring duplicate directory
"C:/new_directory/MSYS2/mingw32/lib/gcc/../../lib/gcc/i686-w64-mingw32/6.1.1/include-fixed"
ignoring duplicate directory
"C:/new_directory/MSYS2/mingw32/lib/gcc/../../lib/gcc/i686-w64-mingw32/6.1.1/../../../../i686-w64-mingw32/include"
ignoring nonexistent directory
"C:/MinGW/MSYS2/mingw32/i686-w64-mingw32/include"
#include "..." search starts here:
#include <...> search starts here:
 C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/include

C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/../../../../include

C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/include-fixed

C:/new_directory/MSYS2/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.1.1/../../../../i686-w64-mingw32/include
 C:/new_directory/MSYS2/mingw32/lib/gcc/../../include/c++/6.1.1

C:/new_directory/MSYS2/mingw32/lib/gcc/../../include/c++/6.1.1/i686-w64-mingw32
 C:/new_directory/MSYS2/mingw32/lib/gcc/../../include/c++/6.1.1/backward
End of search list.
GNU C++14 (gcc-6-branch HEAD with MCF thread model, built by LH_Mouse.) version
6.1.1 20160511 (i686-w64-mingw32)
        compiled by GNU C version 6.1.1 20160511, GMP version 6.1.0, MPFR
version 3.1.4, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: eacb6c45d10560d8e7170e07a095317b
In file included from <stdin>:1:0:
C:/new_directory/MSYS2/mingw32/include/c++/6.1.1/cstdlib:75:25: fatal error:
stdlib.h: No such file or directory
 #include_next <stdlib.h>
                         ^
compilation terminated.

```