Re-commit "[cmake] Enable zlib support on windows"

This recommits r319533 which was broken llvm-config --system-libs
output.  The reason was that I used find_libraries for searching for the
z library.  This returns absolute paths, and when these paths made it
into llvm-config, it made it produce nonsensical flags.  To fix this, I
hand-roll a search for the library in the same way that we search for
the terminfo library a couple of lines below.

This is a bit less flexible than the find_library option, as it does not
allow the user to specify the path to the library at configure time
(which is important on windows, as zlib is unlikely to be found in any
of the standard places cmake searches), but I was able to guide the
build to find it with appropriate values of LIB and INCLUDE environment
variables.

Reviewers: compnerd, rnk, beanz, rafael

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D40779

llvm-svn: 319751

0 files changed, 0 insertions, 0 deletions