Merge #862: Autoconf improvements

3c15130709da26a6d2f25a483aa45e14bf1e4feb Improve CC_FOR_BUILD detection (Tim Ruffing)
47802a476246b67360bc24df78fe5fad6b93c296 Restructure and tidy configure.ac (Tim Ruffing)
252c19dfc654dbb10a35579fa36edb3466904758 Ask brew for valgrind include path (Tim Ruffing)

Pull request description:

  See individual commit messages. These are improvements in preparation of the switch to Cirrus CI. (Maybe I'll just open a PR on top of this one.)

  The first commit made the difference between successful build https://cirrus-ci.com/task/6740575057608704 and unsuccessful build https://cirrus-ci.com/task/4909571074424832.

  I've tested the second commit without cross-compilation and with cross-compilation for android (https://github.com/bitcoin-core/secp256k1/issues/621#issuecomment-495703399)

  When working on the autoconf stuff, I noticed two things that I just want to write down here:
   - At some point we should update [build-aux/m4/ax_prog_cc_for_build.m4](https://www.gnu.org/software/autoconf-archive/ax_prog_cc_for_build.html). This is outdated, and [there have been a lot of fixes](https://github.com/autoconf-archive/autoconf-archive/pull/207) But the latest version is [broken](https://lists.gnu.org/archive/html/autoconf-archive-maintainers/2020-06/msg00002.html), so now is probably not the time.
   - The latest autoconf 2.70 deprecates `AC_PROG_CC_C89`. It's not needed anymore because `AC_PROG_CC` cares about testing for version support. This makes autoconf 2.70 output a warning that we should probably just ignore. We don't want to force users onto 2.70...

ACKs for top commit:
  sipa:
    utACK 3c15130709da26a6d2f25a483aa45e14bf1e4feb
  jonasnick:
    utACK 3c15130 makes sense (with my very basic understanding of autoconf)

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