The ONLY clause works only with MPI versions that have all MPI subroutines
declared in the module. According to Thomas Jahns, this is not always the case
(see comments in #4 for more details).

This way, we avoid target-specific flags '-DUSE_MPI', which result into
target-specific rules that do not use Libtool for the compilation.

We need all this because the configure script accepts only absolute paths for MPI_LAUNCH.

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.

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.

Unify PPM CORE location interface and check whether the library is really available in the configure script.