Add USES=compiler:c11 to fix build on GCC-based architectures:

/usr/local/include/gnutls/gnutls.h:482: error: expected identifier or '(' before '/' token

While here, also add USES=gnome localbase:ldflags.

PR: 237037
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)