Add Legion-specific build flags

cc @manopapad

Add Legion_CXX_FLAGS, Legion_CUDA_FLAGS, Legion_FC_FLAGS, and Legion_LINKER_FLAGS to allow users to set language-specific flags for Legion builds without needing to override or overwrite the global CMake variants.

A motivating example:

A downstream library which depends on (and builds Legion through cmake), would like to build with -Wall -Werror. However the Legion build fails, but the library wants to keep the flags for it's own build. Ideally, it just wants to do -DCMAKE_CXX_FLAGS='-Wall -Werror' -DLegion_CXX_FLAGS='-Wno-error', but it can't and instead has to pull out the flags within its own CMakeLists.txt before configuring Legion.

Or perhaps this library has users which run on particularly esoteric or broken systems, and all they need to do is pass --foo-bar=baz to Legion to get it to compile/link/frobnicate.