GCC 10 and clang 12 as default compilers for Linux and macOS

These compilers will become the default for building packages in conda-forge. One notable change in gcc 10 is that the -fopenmp`` flag in FFLAGSis dropped. In clang 12,-std=c++14explicit flag has been dropped fromCXXFLAGS, as it is the default compilation mode for clang 12. In gcc 11, the default is -std=gnu++17`. In clang>=12 and gcc>=11, we will not provide an explicit C++ standard, and will defer to the compiler default.