portable: sync CFLAGS with upstream Due to the way the Makefiles are structured, there is no direct mapping between upstream and how -portable provides the same functionality. Therefore, add -Wunused-variable and -Wwrite-strings as per previous commits.

portable: propagate AM_CFLAGS/AM_CPPFLAGS Allow for warnings via AM_CFLAGS in a common Makefile, and let those values propagate to all other Makefiles. Currently the more useful checks are disabled until upstream changes are made to reduce warnings. While here, also fix the version string to remove quotes.