[build] Fix race condition during libgcc build

[Eric Botcazou]

Hi,

we have some new machines which seem to be very good at stumbling upon race 
conditions during bootstrap.  Another example with libgcc:

/azun.a/gnatmail/sandbox/gnatcross-cont/x86-linux/gcc/build/./gcc/xgcc -
B/azun.a/gnatmail/sandbox/gnatcross-cont/x86-linux/gcc/build/./gcc/ -
B/azun.a/gnatmail/sandbox/gnatcross-cont/x86-linux/gcc/pkg/i686-pc-linux-
gnu/bin/ -B/azun.a/gnatmail/sandbox/gnatcross-cont/x86-linux/gcc/pkg/i686-pc-
linux-gnu/lib/ -isystem /azun.a/gnatmail/sandbox/gnatcross-cont/x86-
linux/gcc/pkg/i686-pc-linux-gnu/include -isystem 
/azun.a/gnatmail/sandbox/gnatcross-cont/x86-linux/gcc/pkg/i686-pc-linux-
gnu/sys-include    -g -O2 -O2  -g -O2 -DIN_GCC    -W -Wall -Wno-narrowing -
Wwrite-strings -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -Wold-
style-definition  -isystem ./include   -fpic -mlong-double-80 -DUSE_ELF_SYMVER 
-g -DIN_LIBGCC2 -fbuilding-libgcc -fno-stack-protector   -fpic -mlong-
double-80 -DUSE_ELF_SYMVER -I. -I. -I../.././gcc -I../../../src/libgcc -
I../../../src/libgcc/. -I../../../src/libgcc/../gcc -
I../../../src/libgcc/../include -I../../../src/libgcc/config/libbid -
DENABLE_DECIMAL_BID_FORMAT -DHAVE_CC_TLS  -DUSE_TLS -o _negdi2_s.o -MT 
_negdi2_s.o -MD -MP -MF _negdi2_s.dep -DSHARED -DL_negdi2 -c 
../../../src/libgcc/libgcc2.c
config.status: linking ../../../src/libgcc/unwind-generic.h to unwind.h
config.status: linking ../../../src/libgcc/config/i386/linux-unwind.h to md-
unwind-support.h
config.status: linking ../../../src/libgcc/config/i386/sfp-machine.h to sfp-
machine.h
config.status: linking ../../../src/libgcc/gthr-posix.h to gthr-default.h
In file included from ../../../src/libgcc/libgcov-interface.c:27:0:
../../../src/libgcc/gthr.h:148:26: fatal error: gthr-default.h: No such file 
or directory
 #include "gthr-default.h"
                          ^
compilation terminated.
Makefile:894: recipe for target '_gcov_reset.o' failed
make[3]: *** [_gcov_reset.o] Error 1
make[3]: *** Waiting for unfinished jobs....

So there is a race condition between the creation of (soft) links by the 
libgcc configure script and the use of these links by the compilation of files 
launched by the Makefile.

Tentative fix attached, tested on a bunch of machines for several native 
platforms, OK for the mainline?


2016-09-16  Eric Botcazou  <ebotcazou@adacore.com>

	* configure.ac: Do not create links, only substitute the filenames.
	* configure: Regenerate.
	* Makefile.in: Assign the substitution results to variables.
	(LIBGCC_LINKS): Define.
	(enable-execute-stack.c): New rule.
	(unwind.h): Likewise.
	(md-unwind-support.h): Likewise.
	(sfp-machine.h): Likewise.
	(gthr-default.h): Likewise.
	Add $(LIBGCC_LINKS) to the prerequisites of all object files and
	unwind.h as prerequisite of install-unwind_h-forbuild.

-- 
Eric Botcazou
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