Get rid of CONFIGURE_SHARED_ARGS; set BUILD_CC/BUILD_CFLAGS to help in some pkgs.

 - CONFIGURE_SHARED_ARGS is gone; shared configure args are now available
   in configure_args, where more values can be appended in templates.
 - Set BUILD_CC/BUILD_CFLAGS to save some typing in packages.
Makefile: bump version to 53.