build: Use {CFLAGS,CPPFLAGS,LDFLAGS}_FOR_BUILD for helper programs.
* configure.ac (CC_FOR_BUILD): Use AX_CC_FOR_BUILD.
* cipher/Makefile.am (gost-s-box): Add
{CFLAGS,CPPFLAGS,LDFLAGS}_FOR_BUILD.
* doc/Makefile.am (yat2m): Likewise.
* m4/ax_cc_for_build.m4: New.- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>