Don't use LLVM_CXX_FLAGS/LLVM_C_FLAGS

Don't use LLVM_CXX_FLAGS/LLVM_C_FLAGS

llvm-config --cxxflags shows CFLAGS used to compile clang. If clang
installed from package and package was build using GCC then it will
contain GCC specific flags. In case of clang installed as RPM packge
-specs= flags also added. There is not point to use them to
compile plugin. For now hardcode flags in clang-plugin/CMakeLists.txt

Add -O2 - we want plugin to work fast even if we to compiling rspamd
code with -O0.