scripts: PARALLELMFLAGS is evil, rename

The reunification of the glibc/eglibc code paths exposed a nasty
bug in the glibc build: use of PARALLELMFLAGS breaks the build.

See the explanations in that bug report against FC6:
https://bugzilla.redhat.com/show_bug.cgi?format=multiple&id=212111

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 COPYING