Fix detectioon of id3lib and mad.

--- plugins/codec_mp3/CMakeLists.txt.orig	2018-02-02 23:44:29 UTC
+++ plugins/codec_mp3/CMakeLists.txt
@@ -30,6 +30,8 @@ IF (WITH_MP3)
 
     INCLUDE(CheckIncludeFileCXX)
 
+    SET(CMAKE_REQUIRED_INCLUDES "${CMAKE_INSTALL_PREFIX}/include")
+
     CHECK_INCLUDE_FILE_CXX(id3/globals.h HAVE_ID3_HEADER_GLOBALS_H)
     IF (HAVE_ID3_HEADER_GLOBALS_H)
         CHECK_INCLUDE_FILE_CXX(id3/tag.h HAVE_ID3_HEADER_TAG_H)
@@ -57,7 +59,10 @@ IF (WITH_MP3)
             HAVE_ID3_LIB
             ${CMAKE_BINARY_DIR}
             ${_try_id3lib}
-            CMAKE_FLAGS -DLINK_LIBRARIES:STRING=id3\;stdc++\;z
+            CMAKE_FLAGS 
+                 "-DINCLUDE_DIRECTORIES=${CMAKE_INSTALL_PREFIX}/include"
+                 "-DLINK_DIRECTORIES=${CMAKE_INSTALL_PREFIX}/lib"
+                 "-DLINK_LIBRARIES:STRING=id3\;stdc++\;z"
             OUTPUT_VARIABLE _out
         )
     ENDIF (HAVE_ID3_HEADERS)
@@ -78,10 +83,11 @@ IF (WITH_MP3)
 
 #############################################################################
 ### check for libmad headers and library                                  ###
-
+    
+    SET(CMAKE_REQUIRED_INCLUDES "${CMAKE_INSTALL_PREFIX}/include")
     CHECK_INCLUDE_FILES(mad.h HAVE_MAD_H)
     IF (HAVE_MAD_H)
-        CHECK_LIBRARY_EXISTS(mad mad_decoder_init "" HAVE_MAD_LIB)
+        CHECK_LIBRARY_EXISTS(mad mad_decoder_init "${CMAKE_INSTALL_PREFIX}/lib" HAVE_MAD_LIB)
     ENDIF (HAVE_MAD_H)
 
     IF (NOT HAVE_MAD_LIB)