Enable more single-thread optimizations in gnulib code

On many systems (esp. BSD ones), building a recent m4 snapshot produces these warnings: CC regex.o In file included from ../../lib/regex_internal.h:57:0, from ../../lib/regex.c:70: ../../lib/r

posix_spawn, posix_spawnp: Fix build error on Minix 3.3

Building a current m4 snapshot on Minix 3.3, there are compilation errors, such as: --- In file included from ../../lib/execute.c:72: ./spawn.h:727:19: error: conflicting types for 'posix_spawn' _GL_FUNCDECL_SYS (posix_spawn, int,