Update from Gnulib

Update from Gnulib

This incorporates:
2020-08-06 libgmp: add <gmp/gmp.h> support
2020-08-06 Consider that clang defines __OPTIMIZE__ like GCC does
2020-08-06 Use __builtin_expect with clang everywhere
2020-08-05 Use __builtin_clz{,l,ll} with clang, also on Windows
2020-08-05 Use __builtin_ctz{,l,ll} and __builtin_ffs{,l,ll} with clang
2020-07-31 _GL_CMP: Improve documentation
2020-07-30 alloca, largefile: sync with Autoconf master
* lib/c++defs.h, lib/cdefs.h, lib/count-leading-zeros.h:
* lib/count-trailing-zeros.h, m4/alloca.m4, m4/gnulib-common.m4:
* m4/largefile.m4, m4/libgmp.m4:
Copy from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.