Set ffmpeg root when building ffmpeg

Author: aashish24

CMake/cdat_modules/ffmpeg_pkg.cmake

@@ -9,5 +9,6 @@ set (nm FFMPEG)
 string(TOUPPER ${nm} uc_nm)
 set(${uc_nm}_VERSION ${${nm}_MAJOR_SRC}.${${nm}_MINOR_SRC})
 set(FFMPEG_SOURCE ${FFMPEG_URL}/${FFMPEG_GZ})
+set(FFMPEG_ROOT ${cdat_EXTERNALS} CACHE PATH "Path to FFMPEG root directory")
 
 add_cdat_package_dependent(FFMPEG "" "" ON "CDAT_BUILD_GRAPHICS" OFF)