Add support for dev-libs/gmp (GNU MP library)

Packages describing “gmp” as local USE flag

Package “gmp” Flag Description
app-editors/emacs Use the GNU multiple precision arithmetic library (dev-libs/gmp) instead of the bundled mini-gmp subset
dev-lang/ghc Use dev-libs/gmp for integer operations (works faster, but an external depend).
dev-java/gnu-classpath Build with GMP backend for java.math.BigInteger

All packages providing a “gmp” USE flag (23)