HDF5: Fix build errors for PGI on Linux

Most of the compiler flags set for HDF5 when not using
MSVC are actually GCC specific.  This removes that underlying
assumption and makes the check for GCC explicit.

Change-Id: Ifc9d45ad09435144dc5a44dafb8d16f0b3fd9ba1
Merge topic 'hdf5-fix-flags-for-non-gcc' into master

a5cee757 HDF5: Fix build errors for PGI on Linux