Fix autotools build

There were two issues:
* ISL's configure generates include/isl/stdint.h, not isl/stdint.h as
  assumed. This is also changed in the CMake build.

* Need to pass --with-int=imath to ISL's configure; the default is gmp. 

Polly's configure has been regenerated due to changing configure.ac

llvm-svn: 240657