integer-gmp: Simplify gmp/configure invocation
integer-gmp: Simplify gmp/configure invocation

There weas lots of historical cruft to be found here. The `export
SHELLOPTS` breaks on NixOS due to bash syntax in the gcc wrapper script.

Reviewers: hvr

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4347

sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc