Fix: Cycles, missing OSL headers in install

Some headers and precompiled shaders were not properly installed.
This seems to be a regression since !153424: our custom FindOSL
used to define OSL_SHADER_DIR. It is not defined by the CMake
configs from the OSL.

Pull Request: https://projects.blender.org/blender/blender/pulls/154576

intern/cycles/kernel/osl/shaders/CMakeLists.txt

@@ -107,6 +107,30 @@ set(SRC_OSL
   node_rgb_to_bw.osl
 )
 
+get_filename_component(OSL_SHADER_HINT ${OSL_COMPILER} DIRECTORY)
+find_path(OSL_SHADER_DIR
+  NAMES
+    stdosl.h
+  HINTS
+    ${OSL_SHADER_HINT}
+    ${OSL_SHADER_HINT}/../
+    ${OSL_ROOT_DIR}
+    ${OSL_HOME_DIR}
+    /usr/share/OSL/
+    /usr/include/OSL/
+  PATH_SUFFIXES
+    share/OSL/shaders
+    shaders
+)
+
+if(NOT EXISTS "${OSL_SHADER_DIR}")
+  if(WITH_STRICT_BUILD_OPTIONS)
+    message(SEND_ERROR "Precompiled OSL shader headers are not found, stopping")
+  else()
+    message(STATUS "Precompiled OSL shader headers are not found, continuing without")
+  endif()
+endif()
+
 # The headers that OSL ships differs per release so we can not
 # hard-code this.
 file(GLOB SRC_OSL_HEADER_DIST ${OSL_SHADER_DIR}/*.h)