Update the test_headers make target slightly

This patch adds:

1.) The ability to test a specific subset of headers by running:
    make HEADERS_TO_TEST="numerics/parsed_function.h base/dof_map.h" test_headers

2.) Header tests in both opt and dbg mode, as opposed to devel mode.
    Since devel mode sets neither -DDEBUG nor -DNDEBUG, it misses both
    "#ifdef DEBUG" and "#ifdef NDEBUG" sections of the code, so
    testing both opt and dbg modes is more comprehensive.

I'm not super familiar with exactly what syntax is allowed in Makefile.am, so maybe @benkirk can take a look, especially at the "?=" stuff for letting users specify what headers to check via the environment.