HDFFV-8933:
Did not provide default values for clang++ options.
Also, applied wrong values for the *_CPPFLAGS.

Solution:
Added default values for *_CXXFLAGS.
Fixed the *_CPPFLAGS values.

Tested: wren with and without --enable-production.


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@25677 dab4d1a6-ed17-0410-a064-d1ae371a2980
Added instruction to run bin/reconfigure after changes.


git-svn-id: https://svn.hdfgroup.uiuc.edu/hdf5/trunk@25679 dab4d1a6-ed17-0410-a064-d1ae371a2980