[cmake] Always respect existing CMAKE_REQUIRED_FLAGS when adding additional ones.

Summary:
Always respect existing CMAKE_REQUIRED_FLAGS when adding
additional ones.  This is important when cross compiling where
--sysroot and -target were already added.

In particular, this is needed when cross compiling from Darwin to
Linux, since --sysroot is required to find headers and libraries.

Cmake has a similar bug in check_include_file[_cxx] where
CMAKE_REQUIRED_LIBRARIES isn't passed, which causes
try_compile to fail.
(please see https://gitlab.kitware.com/cmake/cmake/merge_requests/1620)

Reviewers: compnerd, silvas, beanz, brad.king

Reviewed By: compnerd

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D41568

llvm-svn: 321434

0 files changed, 0 insertions, 0 deletions