c-strtold: Avoid link error on AIX 7. * lib/c-strtod.c: Test also HAVE_STRTOD_L or HAVE_STRTOLD_L. * m4/c-strtod.m4 (gl_C_STRTOD): Test whether strtod_l exists. (gl_C_STRTOLD): Test whether strtold_l exists. Reported by Albert Chin <bug-gnulib@mlists.thewrittenword.com>.