build: Use {CFLAGS,CPPFLAGS, LDFLAGS}_FOR_BUILD for helper programs
* doc/Makefile.am: add CPPFLAGS_FOR_BUILD for yat2m-for-build (other flags were already present). * src/Makefile.am: add {CFLAGS,CPPFLAGS,LDFLAGS}_FOR_BUILD for mkhelper, mkw32errmap, and mkerrcodes.