Add a simple check for mpv/qthelper.hpp

Some people have failing builds because they have a old version of mpv
that doesn't include the qthelper header. I have added a simple check
for this. Since there is no reliable way for us to currently check for
a specific version of mpv (constant flux) we might want to add checks
for some specific functionality instead.

CMakeModules/PlayerConfiguration.cmake

@@ -12,5 +12,9 @@ else(NOT OPENGL_FOUND)
 endif(NOT OPENGL_FOUND)
 
 find_package(MPV REQUIRED)
-
 include_directories(${MPV_INCLUDE_DIR})
+
+find_path(MPV_QT_HELPER mpv/qthelper.hpp HINTS ${MPV_INCLUDE_DIR})
+if(${MPV_QT_HELPER} STREQUAL "MPV_QT_HELPER-NOTFOUND")
+  message(FATAL_ERROR "Missing mpv/qthelper.h - maybe your mpv version is to old?")
+endif(${MPV_QT_HELPER} STREQUAL "MPV_QT_HELPER-NOTFOUND")