Libtool nocommon overrides

For GCC 10, a new type of override has been introduced: NOCOMMON_OVERRIDE_LIBTOOL

This override is intended to help fix certain build system problems arising from using older libtool versions in packages. Details can be found in #484, contributed by @nivedita76 and @pchome. Essentially, two variables don't get set properly when -flto and -fno-common are used together. NOCOMMON_OVERRIDE_LIBTOOL is a hack to set them ourselves. Define NOCOMMON_OVERRIDE_LIBTOOL=yes to activate it per-package or globally via make.conf (not recommended).