mbsstr, mbscasestr, mbspcasecmp, unistr, unigbrk: Simplify. Suggested by Paul Eggert. * lib/string.in.h (mbsstr, mbspcasecmp, mbscasestr): On ISO C compliant compilers, use the _Generic based macro instead of the 'typeof' based macro. * lib/unistr.in.h (u*_check, u*_next, u*_prev, u*_chr, u*_strchr, u*_strrchr, u*_strpbrk, u*_strstr): Likewise. * lib/unigbrk.in.h (u*_grapheme_next, u*_grapheme_prev): Likewise.