[Boilerplate] Only depend on GMP if user has specified -DENABLE_GMP

The finite element spaces based on the OPBFiniteElementMap in the
boilerplate layer use #ifdef HAVE_GMP to switch on basis orthonormalization
using GMP. That is problematic if GMP was found, but the user has not
added the GMP_FLAGS to their program: HAVE_GMP will be defined to
ENABLE_GMP in that case, so GMP will be used in the boilerplate code,
but the GMP support in dune-common requires ENABLE_GMP to be defined to
1 to kick in, causing compilation errors even if the user only needs a
different space from boilerplate.

Fixed by replacing #ifdef HAVE_GMP with #if HAVE_GMP.