build: Fix LLVM find package picking up system-wide libraries (AcademySoftwareFoundation#1866)

This change fixes LLVM find package ignoring the library path provided
by LLVM_CONFIG and picking up a system-wide library instead.

This happens when a custom LLVM is compiled statically, and there is a
system-wide LLVM of the version which OSL expects.

The solution is to pass NO_DEFAULT_PATH to the find_library so that
the function only considers paths explicitly coming from the LLVM's
configuration executable.

It is a bit unclear what is the expected behavior of this module when
LLVM_CONFIG is not available. It is possible to limit possible side
effects of this change by only ignoring default paths if the LLVM
CONFIG executable is found.

In practice this issue happened in Blender's build environment where
HIP is used for OpenImageDenoiser: HIP toolchain pulls in LLVM system
wide. While it is not an issue on itself (as it is only a compile time
dependency for HIP support in OIDN) in conflicts with the logic in the
OSL's find_package(LLVM).

Signed-off-by: Sergey Sharybin <sergey@blender.org>

Author: sergeyvfx

src/cmake/modules/FindLLVM.cmake

@@ -97,10 +97,12 @@ string (REPLACE " " ";" LLVM_SYSTEM_LIBRARIES "${LLVM_SYSTEM_LIBRARIES}")
 
 find_library ( LLVM_LIBRARY
                NAMES LLVM-${LLVM_VERSION} LLVM
-               PATHS ${LLVM_LIB_DIR})
+               PATHS ${LLVM_LIB_DIR}
+               NO_DEFAULT_PATH)
 find_library ( LLVM_MCJIT_LIBRARY
                NAMES LLVMMCJIT
-               PATHS ${LLVM_LIB_DIR})
+               PATHS ${LLVM_LIB_DIR}
+               NO_DEFAULT_PATH)
 
 if (NOT LLVM_LIBRARY)
     # if no single library was found, use llvm-config to generate the list
@@ -118,8 +120,9 @@ execute_process (COMMAND ${LLVM_CONFIG} --shared-mode
        OUTPUT_STRIP_TRAILING_WHITESPACE)
 if (LLVM_VERSION VERSION_GREATER_EQUAL 9.0 AND (LLVM_SHARED_MODE STREQUAL "shared"))
     find_library ( _CLANG_CPP_LIBRARY
-		  NAMES "clang-cpp"
-		  PATHS ${LLVM_LIB_DIR})
+                   NAMES "clang-cpp"
+                   PATHS ${LLVM_LIB_DIR}
+                   NO_DEFAULT_PATH)
     if (_CLANG_CPP_LIBRARY)
         list (APPEND CLANG_LIBRARIES ${_CLANG_CPP_LIBRARY})
     endif ()
@@ -131,7 +134,8 @@ foreach (COMPONENT clangFrontend clangDriver clangSerialization
                    clangSupport clangAPINotes)
     find_library ( _CLANG_${COMPONENT}_LIBRARY
                   NAMES ${COMPONENT}
-                  PATHS ${LLVM_LIB_DIR})
+                  PATHS ${LLVM_LIB_DIR}
+                  NO_DEFAULT_PATH)
     if (_CLANG_${COMPONENT}_LIBRARY)
         list (APPEND CLANG_LIBRARIES ${_CLANG_${COMPONENT}_LIBRARY})
     endif ()