* GCC: constrain version 6, 7, and 8 to build with ISL version at
most 0.18
* GCC version 9 (not yet released) will be compatible with
ISL version 0.20 so future GCC releases are constrained to build
with ISL version at most 0.20
* ISL: Add v0.15, replace all md5 sums with sha256 sums for ISL
versions
* GCC versions before 6 were constrained to build with ISL 0.14
but have been confirmed to work with ISL 0.15
* GCC: Place an upper bound on GCC's mpfr dependency
33b65ecca0