mkc_check_custom: new option -e was added
mkc_check_custom: additional options to compiler are passed via CARGS env. variable
mkc_check_custom: document option -e; minor improvements for -m
New variable MKC_CHECK_CC_OPTS was added
Include Makefile.{inc,common} after system variables are set, including CC and CXX
New variable MKC_CHECK_CXX_OPTS
Fix for previous commits