Improve CC_FOR_BUILD detection

Improve CC_FOR_BUILD detection

This commits simply uses CC as CC_FOR_BUILD and the same for
corresponding flags if we're not cross-compiling. This has a number of
benefits in this common case:
 - It avoids strange cases where very old compilers are used (#768).
 - Flags are consistently set for CC and CC_FOR_BUILD.
 - ./configure is faster.
 - You get compiler x consistently if you set CC=x; we got this wrong
   in CI in the past.

./configure warns if a _FOR_BUILD variable is set but ignored because
we're not cross-compiling.

The change exposed that //-style comments are used in gen_context.c,
which is also fixed by this commit.

This commit also reorganizes code in configure.ac to have a cleaner
separation of sections.