Fix makefile var, GMP_H_FILE.

It now uses the absolute path instead of the relative path, which is not useful
from third-party/Makefile. Also update RE2_H_FILE var to use include dir var,
for consistency.
Merge pull request #831 from thomasvandoren/fix-gmp-re2-h-file-vars

Fix makefile var, GMP_H_FILE.

It now uses the absolute path instead of the relative path, which is not useful
from third-party/Makefile. Also update RE2_H_FILE var to use include dir var,
for consistency.

This corrects a mistake made in #814 .