Generate gitversion.h in autoconf builds

Generate gitversion.h in autoconf builds

Add a custom makefile rule to generate lib/External/isl/gitversion.h
from GIT_HEAD_ID and trigger it using BULIT_SOURCES to ensure the file
exists before compilation starts.

The latest ISL creates gitversion.h from Makefile.am only, instead also
from configure.ac in previous version. While the Polly build invokes
configure, it does not invoke ISL's make such that the file was missing.

Invoking ISL's make would come with additional problems such as
triggering automake because of not preserved file time stamps.
Re-running automake might not be successful on other system
configurations for instance because it was preconfigured without
--with-clang option.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@247142 91177308-0d34-0410-b5e6-96231b3b80d8