The compatibility layer disabled MPI whenever it did not find
the --enable-parallel flag. This is overeager and harmful if users
are gradually switching to replacing the configure flags with real
cmake flags.

Consider an opts file, with the following content:
CMAKE_FLAGS="-DUSE_MPI=ON"

The compatibility layer would now parse the non-existent CONFIGURE_FLAGS
variable for an --enable-parallel option and disable MPI because it was
not found. The -DCMAKE_DISABLE_FIND_PACKAGE_MPI=TRUE supersedes the USE_MPI=ON
and we get a sequential build.

The fix applied in this patch removes the else clause and only adds
cmake flags when the --enable-parallel flag was found. To still explicitly
allow to force sequential builds, the --disable-parallel flag is parsed
in the same manner.