Configure script prefers clang for macOS systems

In the previous commit (Commit 1bc00c9: Only using clang in macOS systems
that also have GCC) we set the used C compiler for high-level programs to
be 'clang' on macOS systems. But I forgot to do the same kind of change in
the configure script (to prefer 'clang' when we are testing for a C
compiler on the host).

With this commit, the compiler checking phases of the configure script have
been improved, so on macOS systems, we now first search for 'clang', then
search for 'gcc'.

While doing this, I also noticed that the 'rpath' checking command was done
before we actually define 'instdir'!!! So in effect, the 'rpath' directory
was being set to '/lib'! So with this commit, this test has been taken to
after defining 'instdir'.

0 files changed, 0 insertions, 0 deletions