autoconf: isl depends on gmp include files

When libgmp header files are not installed in the default /usr/include location,
configure used to fail to find the gmp headers when testing for isl/ast.h.  This
patch adds the gmp include path to the compiler flags before testing for ISL.

Merged from https://llvm.org/svn/llvm-project/polly/trunk@168090

llvm-svn: 170725