Currently, the libomp CMake build system uses a Perl script to configure files
(tools/expand-vars.pl). This patch replaces the use of the Perl script by using
CMake's configure_file() function. The major changes include:
1. *.var has every $KMP_* variable changed to @LIBOMP_*@
2. kmp_config.h.cmake is a new file which contains all the feature macros and
   #cmakedefine lines
3. Most of the -D lines have been moved from LibompDefinitions.cmake but some
   OS specific MACROs (e.g., _GNU_SOURCE) remain.
4. All expand-vars.pl related logic is removed from the CMake files.

One important note about this change is that it breaks the old Perl+Makefile
build system because it can't create kmp_config.h properly.

Differential Review: http://reviews.llvm.org/D12211

llvm-svn: 246314