Eliminate CLEARENV after configure.mk migration
Move more Makefile variables into configure.mk
Add MAKEFILE and MAKEOVERRIDES to configure.mk.org.
Remove more configure.mk variables from build.mk
Fix CC in FIPS_BUILD_CMD and FIPS_CRYPTO_BUILD_CMD

This should have been $(CC) all along; removing $(BUILDENV) from the
make invocation causes ${{CC}} to be empty, thanks to 'make -e', triggering a
build failure as LDCMD receives no value.
Eliminate BUILDENV after configure.mk migration