configure: Add a test for the minimum compiler version

configure: Add a test for the minimum compiler version

So far we only had implicit requirements for the minimum compiler version,
e.g. we require at least GCC 4.1 for the support of atomics. However,
such old compiler versions are not tested anymore by the developers, so
they are not really supported anymore. Since we recently declared explicitly
what platforms we intend to support, we can also get more explicit on the
compiler version now. The supported distributions use the following version
of GCC:

      RHEL-7: 4.8.5
      Debian (Stretch): 6.3.0
      Debian (Jessie): 4.8.4
      OpenBSD (ports): 4.9.4
      FreeBSD (ports): 8.2.0
      OpenSUSE Leap 15: 7.3.1
      Ubuntu (Xenial): 5.3.1
      macOS (Homebrew): 8.2.0

So we can safely assume GCC 4.8 these days. For Clang, the situation is
a little bit more ambiguous, since it is sometimes not available in the
main distros but rather third party repositories. At least Debian Jessie
uses version 3.5, and EPEL7 for RHEL7 uses 3.4, so let's use 3.4 as
minimum Clang version now - we still can adjust this later if necessary.

Unfortunately Apple uses different version numbers for the Clang that is
included in their Xcode suite, so we need to check the version numbers
for Xcode separately. Xcode 5.1 seems to be the first one that has been
shipped with LLVM 3.4, so use this version as the minimum there.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>