build: Use {CFLAGS,CPPFLAGS,LDFLAGS}_FOR_BUILD for helper programs.

* src/Makefile.am: pass distinct build flags to CC_FOR_BUILD.

We don't want to use the standard CFLAGS, CPPFLAGS, and LDFLAGS for
build helper tools because when cross-compiling they might include
choices that only make sense for the platform we are aiming to build
for.

That said, the environment might offer standard build flags for
non-cross-built helper tools too. So we include those flags in the
right place here.

• Debian-bug-id: #932213
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>