Configuration: Checking that C and Fortran compilers work

Until now, we were just checking for the existance of a C and Fortran
compiler. But it can happen that even if they exist, they don't operate
properly, for example see some errors that have been reported until now in
P.S. (both on different macOS systems). But finding this source after the
programs have started is frustrating for the user.

With this commit, before we start building anything, we'll check these two
compilers with a simple program and see if they can indeed compile, and if
their compiled program can run. If it doesn't work an elaborate error
message is printed to help the users navigate to a solution.

Also, the building of `flock' within `configure.sh' has been moved just
before calling `basic.mk'. This was done so any warning/error message
is printed before actually building anything.

This fixes bug #56715.

P.S. The error messages:

C compiler
----------
conftest.c:9:19: fatal error: stdio.h: No such file or directory
                   ^
compilation terminated.
----------

Fortran compiler
----------------
dyld: Library not loaded: @rpath/libisl.10.dylib
Referenced from:
/path/to/anaconda2/gcc/libexec/gcc/x86_64-apple-darwin15.5.0/4.9.3/f951
Reason: image not found
gfortran: internal compiler error: Abort trap: 6
----------------

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