math/suitesparse: simplify

• Allow using Clang again (base libomp.so or OpenMP disabled)
• Respect C++ library if using GCC (must be libc++ on Clang architectures)
• Respect CFLAGS/CXXFLAGS in subdirectories using CMake
• Respect BLAS/LAPACK choice (actually use OpenBLAS)
• Enable Intel threading building blocks support
• Switch to vendor build/install targets
• Adjust plist for new items (GraphBLAS, Mongoose, DOCS)
• Skip running demos during build

PR: 240899
Tested by: pkubaj
Approved by: fortran (thierry)
MFH: 2019Q4