_exit.2, abs.3, acos.3, acosh.3, asin.3, asinh.3, atan.3, atan2.3, atanh.3, atoi...

_exit.2, abs.3, acos.3, acosh.3, asin.3, asinh.3, atan.3, atan2.3, atanh.3, atoi.3, cbrt.3, ceil.3, copysign.3, cos.3, cosh.3, div.3, erf.3, erfc.3, exp.3, exp2.3, expm1.3, fabs.3, fdim.3, finite.3, floor.3, fma.3, fmax.3, fmin.3, fmod.3, fpclassify.3, frexp.3, fwide.3, hypot.3, ilogb.3, isalpha.3, isgreater.3, iswblank.3, ldexp.3, lgamma.3, log.3, log10.3, log1p.3, log2.3, logb.3, lrint.3, lround.3, modf.3, nan.3, nextafter.3, pow.3, printf.3, remainder.3, remquo.3, rint.3, round.3, scalbln.3, scanf.3, signbit.3, sin.3, sinh.3, sqrt.3, strtod.3, strtol.3, strtoul.3, tan.3, tanh.3, tgamma.3, trunc.3, wprintf.3: Remove "or 'cc -std=c99'" from SYNOPSIS

Under the FTM requirements all of these pages document the
requirement for _ISOC99_SOURCE. And feature_test_macros(7) now
documents that "cc -std=c99" produces the same effect as defining
_ISOC99_SOURCE. So, all of these pages don't additionally need
to specify "or 'cc -std=c99'" under the FTM requirements
in the SYNOPSIS. Removing that redundant text also simplifies
the SYNOPSIS a little.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>