[PATCH][DRIVER] Wrong C++ include paths when configuring with "--with-sysroot=/"

[Pavel Kopyl]

Hi all,


To build a GCC-4.9.2 ARM cross-compiler for my setting I need to 
configure it with  "--with-sysroot=/ 
--with-gxx-include-dir=/usr/include/c++/4.9.2".
But I found that gcc driver removes the leading slash from resulting paths:

`gcc -print-prog-name=cc1plus` -v
...
ignoring nonexistent directory "usr/include/c++/4.9.2"   <- HERE
ignoring nonexistent directory 
"usr/include/c++/4.9.2/armv7l-tizen-linux-gnueabi"   <- AND HERE
ignoring nonexistent directory "usr/include/c++/4.9.2/backward" <- AND HERE
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/armv7l-tizen-linux-gnueabi/4.9.2/include
/usr/local/include
/usr/lib/gcc/armv7l-tizen-linux-gnueabi/4.9.2/include-fixed
/usr/include

It's also reproducible on trunk.

Attached patch fixes this bug.

Thanks,
Pavel.
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