We have to have this as long as we distribute the automatically generated
Autotools and othe files via the git repository.

This allows for sanitization of the build environment on the command line.

For example:
  export BUILD_CC='bad stuff'
  ./configure 'BUILD_CC='
  make

Remove declaration of the 'BUILD_C_INCLUDE' and 'BUILD_FC_INCLUDE' precious variables from the configure script since they are not used.

The shortcut for the ISO C99 test, which simplifies the implementation, is copied from PPM.

This reverts commit c4fade36.

This makes the branch identical to 5ebb81ec.

The idea is that when an additional feature is enabled, we should run the same
tests as without it plus additional tests for the feature.

This is the first step towards building serial versions of tests also when MPI is enabled.

This is the first step towards switching to the parallel test harness.

The configure options enabling the interfaces are named consistently with the
options enabling Fortran interfaces.