The AX_PROG_CC_FOR_BUILD implementation to find a native CC compiler is
slightly better because it uses AC_PROG_CC and AC_PROG_CPP to find the
native compiler instead of just defaulting to `gcc` as AX_CC_FOR_BUILD
does.
AX_PROG_CC_FOR_BUILD also sets BUILD_EXEEXT that we already use in the
Makefile.am for `lib/dns/gen` while AX_CC_FOR_BUILD uses
EXEEXT_FOR_BUILD.
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