As per the GNU Make docs:

• Linker arguments to be passed to -L can be set via LDFLAGS and those passed to -l can be set via LDLIBS. Currently, we only set LDFLAGS during the extension build.
• Extra args to the C++ compiler are set via the CXXFLAGS variable. Currently, we only set CFLAGS which only affects how the libbson wrapper/bindings are compiled but doesn't affect compilation of pymongoarrow.lib which is a C++ extension.