[llvm] [cmake] Use pkg-config to obtain libffi search hints

Extend `FindFFI.cmake` to include the paths obtained from pkg-config
when searching for libffi.  This is going to help systems where libffi
is installed in nonstandard directory such as Gentoo, saving us from
having to copy the paths from pkg-config to `FFI_*` variables
explicitly.  The logic is inspired by `FindLibEdit.cmake`.

Author: mgorny

llvm/cmake/modules/FindFFI.cmake

@@ -23,7 +23,10 @@
 # Additionally, the following import target will be defined:
 # FFI::ffi
 
-find_path(FFI_INCLUDE_DIRS ffi.h PATHS ${FFI_INCLUDE_DIR})
+find_package(PkgConfig QUIET)
+pkg_check_modules(PC_LIBFFI QUIET libffi)
+
+find_path(FFI_INCLUDE_DIRS ffi.h PATHS ${FFI_INCLUDE_DIR} ${PC_LIBFFI_INCLUDE_DIRS})
 if( EXISTS "${FFI_INCLUDE_DIRS}/ffi.h" )
   set(FFI_HEADER ffi.h CACHE INTERNAL "")
   set(HAVE_FFI_H 1 CACHE INTERNAL "")
@@ -35,8 +38,8 @@ else()
   endif()
 endif()
 
-find_library(FFI_LIBRARIES NAMES ffi PATHS ${FFI_LIBRARY_DIR})
-find_library(FFI_STATIC_LIBRARIES NAMES libffi.a PATHS ${FFI_LIBRARY_DIR})
+find_library(FFI_LIBRARIES NAMES ffi PATHS ${FFI_LIBRARY_DIR} ${PC_LIBFFI_LIBRARY_DIRS})
+find_library(FFI_STATIC_LIBRARIES NAMES libffi.a PATHS ${FFI_LIBRARY_DIR} ${PC_LIBFFI_LIBRARY_DIRS})
 
 if(FFI_LIBRARIES)
   include(CMakePushCheckState)