Update CMakeLists.txt to include yuvconstants tool and deps on gflags

Bug: None
Change-Id: I5658addc2c44e6cabfcc8d349e66e13da6c3c0e6
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/3928147
Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Wan-Teh Chang <wtc@google.com>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5f0306b..ea45a5e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,6 +41,9 @@
 ADD_EXECUTABLE			( yuvconvert ${ly_base_dir}/util/yuvconvert.cc )
 TARGET_LINK_LIBRARIES	( yuvconvert ${ly_lib_static} )
 
+# this creates the yuvconstants tool
+ADD_EXECUTABLE      ( yuvconstants ${ly_base_dir}/util/yuvconstants.c )
+TARGET_LINK_LIBRARIES  ( yuvconstants ${ly_lib_static} )
 
 find_package ( JPEG )
 if (JPEG_FOUND)
@@ -78,6 +81,12 @@
   if(NACL AND NACL_LIBC STREQUAL "newlib")
     target_link_libraries(libyuv_unittest glibc-compat)
   endif()
+
+  find_library(GFLAGS_LIBRARY gflags)
+  if(NOT GFLAGS_LIBRARY STREQUAL "GFLAGS_LIBRARY-NOTFOUND")
+    target_link_libraries(libyuv_unittest gflags)
+    add_definitions(-DLIBYUV_USE_GFLAGS)
+  endif()
 endif()