gmp-4.2.1: Set CC_FOR_BUILD to ensure the host system compiler is used, not the targe...

gmp-4.2.1: Set CC_FOR_BUILD to ensure the host system compiler is used, not the target one

This addresses errors like:

| NOTE: make -j 16
| i586-poky-linux-gcc  -m32   -march=i586 --sysroot=/srv/home/pokybuild/yocto-autobuilder/yocto-slave/nightly-non-gpl3/build/build/tmp/sysroots/qemux86 `test -f 'gen-fac_ui.c' || echo './'`gen-fac_ui.c -o gen-fac_ui
| i586-poky-linux-gcc  -m32   -march=i586 --sysroot=/srv/home/pokybuild/yocto-autobuilder/yocto-slave/nightly-non-gpl3/build/build/tmp/sysroots/qemux86 `test -f 'gen-fib.c' || echo './'`gen-fib.c -o gen-fib
| i586-poky-linux-gcc  -m32   -march=i586 --sysroot=/srv/home/pokybuild/yocto-autobuilder/yocto-slave/nightly-non-gpl3/build/build/tmp/sysroots/qemux86 `test -f 'gen-bases.c' || echo './'`gen-bases.c -o gen-bases -lm
| i586-poky-linux-gcc  -m32   -march=i586 --sysroot=/srv/home/pokybuild/yocto-autobuilder/yocto-slave/nightly-non-gpl3/build/build/tmp/sysroots/qemux86 `test -f 'gen-psqr.c' || echo './'`gen-psqr.c -o gen-psqr -lm
| ./gen-fac_ui 32 0 >mpz/fac_ui.h || (rm -f mpz/fac_ui.h; exit 1)
| ./gen-bases header 32 0 >mp_bases.h || (rm -f mp_bases.h; exit 1)
| ./gen-bases table 32 0 >mpn/mp_bases.c || (rm -f mpn/mp_bases.c; exit 1)
| ./gen-fib header 32 0 >fib_table.h || (rm -f fib_table.h; exit 1)
| ./gen-bases: error while loading shared libraries: libgcc_s.so.1: cannot open shared object file: No such file or directory
| ./gen-bases: error while loading shared libraries: libgcc_s.so.1: cannot open shared object file: No such file or directory
| ./gen-fib table 32 0 >mpn/fib_table.c || (rm -f mpn/fib_table.c; exit 1)
| make: *** [mp_bases.h] Error 1

and matches the fix for the other gmp version.

[YOCTO #2992]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>