improve M4 quoting

improve M4 quoting

* configure.ac: Add quotes.
* gl/m4/mgetgroups.m4 (gl_MGETGROUPS): Likewise.
* m4/boottime.m4 (GNULIB_BOOT_TIME): Likewise.
* m4/check-decl.m4 (gl_CHECK_DECLS): Likewise.
* m4/gmp.m4 (cu_GMP): Likewise.
* m4/jm-macros.m4 (coreutils_MACROS, gl_CHECK_ALL_HEADERS): Likewise.
(gl_CHECK_ALL_TYPES): Likewise.
* m4/lib-check.m4 (cu_LIB_CHECK): Likewise.
* m4/stat-prog.m4 (cu_PREREQ_STAT_PROG): Likewise.

FYI, I first ran this command:
git ls-files | grep -E '\.(m4|ac)$' | xargs perl -pi \
-e 's/(AC_[A-Z_]+\()([^[()]+?)([,)])/$1\[$2]$3/g;' \
-e 's/(AC_[A-Z_]+\(\[[^,]+?\], )([^,[()]+?)([,)])/$1\[$2]$3/g;' \
-e 's/(AC_[A-Z_]+\((?:\[[^,]+?\], ){2})([^,[()]+?)([,)])/$1\[$2]$3/g'
Then I updated serial numbers and copyright dates manually.
Also, I manually added two pairs of quotes in boottime.m4.