This adds macOS to the travis so we can test all the same configurations
under macOS, both with Apple's clang and proper gcc on macOS.
It also runs the ctime test, and valgrind on the tests just like on
linux.

The current travis script is pretty messy because we added more and more
configuration over time each one required more complex ifs making it
harder to read, and because of the somewhat complex logic it failed[1]
on macOS(having an old bash version) so I decided to just throw it into
a standalone sh script instead, that way it can be formatted nicely and
with -e it will fail on every error without needing to put &&
everywhere.

Some changed in the script:

    Replaced the usage of libtool with the locally generated libtool
(#723 (comment))

    Added --error-exitcode=42 to the ctime tests because they currently
silently fail on -O0
(https://travis-ci.org/github/bitcoin-core/secp256k1/jobs/681571334#L454)
and disabled the ctime tests on -O0.

    Moved the valgrind tests to the matrix so that they'll run on both
gcc and clang and on macOS.
    (also, now that #710 is merged we always pass -DVALGRIND when the
valgrind headers exist but I left the explicit CFLAGS in those tests
anyway, there's no harm in explicitly doing that)

    Removed the use of EXTRAFLAGS for setting CFLAGS, it's enough to
just set CFLAGS directly and it can cause troubles in sh (the whole
EXTRAFLAGS="--disable-openssl-tests CPPFLAGS=-DVALGRIND")

    We have to explicitly set the gcc version on macOS+gcc because macOS
ship with a fake gcc which is basically just an alias to their clang
compiler, and installing proper gcc from brew adds a gcc-* binary and
doesn't replace the gcc binary, so we have to explicitly set CC=gcc-9
under that scenario, so I also explicitly install gcc@9 so it shouldn't
break when macOS gets gcc-10.

    Bumped ubuntu to bionic because of #748 (comment) (the end of End of
Standard Support is in a year anyway) it's in a separate commit so that
if anyone have concerns I'll just drop that commit.

    https://travis-ci.org/github/elichai/secp256k1/jobs/681663742#L336