gentoo-overlay/media-gfx/darktable/files/darktable-find-opencl-header.patch

diff -Nur a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt	2017-12-24 09:09:10.000000000 +0000
+++ b/CMakeLists.txt	2017-12-25 20:42:40.998693994 +0000
@@ -355,7 +355,7 @@
       message(STATUS "Found clang compiler - ${CLANG_OPENCL_COMPILER}")
 
       find_path(CLANG_OPENCL_INCLUDE_DIR opencl-c.h
-        HINTS ${LLVM_INSTALL_PREFIX}/lib/clang ${LLVM_INSTALL_PREFIX}/lib64/clang
+        HINTS ${LLVM_INSTALL_PREFIX}/lib/clang ${LLVM_INSTALL_PREFIX}/lib64/clang /usr/lib/clang /usr/lib64/clang
         PATH_SUFFIXES include ${LLVM_PACKAGE_VERSION}/include
         NO_DEFAULT_PATH
       )