GCCcore/9.3.0 fails to find GMP in stage2 for ISL on OpenSUSE Leap 15.2

[zao]

After fixing binutils on OpenSUSE Leap 15.2 I'm trying to build the full foss/2020a toolchain.

During the build of GCCcore/9.3.0 while configuring ISL for stage 2 with the following run_cmd command, it finds the header but not the static library in the stage2 prefix:

./configure --prefix=/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff --with-pic --disable-shared --with-gmp=system --with-gmp
-prefix=/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff  --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu

checking which gmp to use... system
checking gmp.h usability... yes
checking gmp.h presence... yes
checking for gmp.h... yes
checking for main in -lgmp... no
configure: error: gmp library not found

The failing config.log test uses this command line:

configure:19076: gcc -o conftest -O3 -fomit-frame-pointer -malign-double -fstrict-aliasing -ffast-math -I/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff/include  -L/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff/lib -I/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff/include  -L/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff/lib  conftest.c -lgmp  -lgmp  >&5 

The given -L/tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff/lib location for GMP is incorrect as on this platform, the library is in a lib64 directory: /tmp/eb-build/GCCcore/9.3.0/system-system/gcc-9.3.0/stage2_stuff/lib64/libgmp.a

@bartoldeman found a part in gcc.py that assumes that libraries are installed in a lib directory, not sure if that's a root cause or just an attempt to plaster over this GCC build behaviour on some platforms:

easybuild-easyblocks/easybuild/easyblocks/g/gcc.py

Lines 595 to 601 in 6e63e14

	if lib == "gmp":
	# make sure correct GMP is found
	libpath = os.path.join(stage2prefix, 'lib')
	incpath = os.path.join(stage2prefix, 'include')
	cppflags = os.getenv('CPPFLAGS', '')
	env.setvar('CPPFLAGS', "%s -L%s -I%s " % (cppflags, libpath, incpath))

[boegel]

@zao So this would need a small change in the GCC easyblock to also consider lib64 if that lib dir was not found, rather than blindly assuming it's there?

[boegel]

@zao Can you take #2339 for a spin on OpenSUSE?