CMake: look for `libebur128`

We are going to use said library to calculate the two measures in question
from decoded audio.

I am adding this in default-on state, since this library
should be well-packaged everywhere, and this is something that,
i presume, we want.

CMakeLists.txt

@@ -326,6 +326,8 @@ if(USE_TAGPARSER)
   pkg_check_modules(TAGPARSER REQUIRED tagparser)
 endif()
 
+pkg_check_modules(LIBEBUR128 IMPORTED_TARGET libebur128)
+
 if(NOT TAGLIB_FOUND AND NOT TAGPARSER_FOUND)
   message(FATAL_ERROR "You need either TagLib or TagParser!")
 endif()