build: Prefix CFLAGS and friends with OSL_.

It is best practice to leave the "official" CPPFLAGS, CFLAGS and
LDFLAGS variables alone and just add them at the end of the command
line options. This way the user may override any of our settings
by setting the corresponding option in the official variable. For
example, to deactivate warnings about unused variables or functions,
one may compile with

	make CFLAGS=-Wno-unused