CMake: Use pkg-config to find vmaf

BUG=aomedia:2748

Change-Id: Ic94f3baed5cd1ce8ff806bc070f830ca8093d48f
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 80b6d27..6969189 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -433,16 +433,22 @@
               ${AOM_ENCODER_TOOL_TARGETS})
 
   if(CONFIG_TUNE_VMAF)
-    find_library(VMAF libvmaf.a vmaf)
-    if(NOT VMAF)
-      message(FATAL_ERROR "VMAF library not found.")
+    find_package(PkgConfig)
+    if(PKG_CONFIG_FOUND)
+      pkg_check_modules(VMAF REQUIRED libvmaf)
+    else()
+      find_library(VMAF VMAF_LIBRARIES)
+      if(NOT VMAF_LIBRARIES)
+        message(FATAL_ERROR "VMAF library not found.")
+      endif()
+      message("-- Found VMAF library: " ${VMAF_LIBRARIES})
     endif()
-    message("-- Found VMAF library: " ${VMAF})
     set_target_properties(aom PROPERTIES LINKER_LANGUAGE CXX)
     if(BUILD_SHARED_LIBS)
       set_target_properties(aom_static PROPERTIES LINKER_LANGUAGE CXX)
     endif()
-    target_link_libraries(aom PRIVATE ${VMAF})
+    target_link_libraries(aom PRIVATE ${VMAF_LIBRARIES})
+    append_compiler_flag("${VMAF_CFLAGS}")
   endif()
 endif()