CMake: remove CUSTOM_(LINK|INCLUDE)_DIR

When some header or library is not found introduce a proper find_* call for it
and use the results. Especially injecting additional link directories would just
confuse the automatic library ordering done by CMake and is basically never the
right solution for any link problem.

CMakeLists.txt

@@ -99,8 +99,6 @@ set(ENV{DOXYGEN_DOCDIR} ${DOXYDOCS})
 set(GENERATED_SRC "${CMAKE_BINARY_DIR}/generated_src" CACHE INTERNAL "Stores generated files")
 set(LIBRARY_OUTPUT_PATH ${PROJECT_BINARY_DIR}/lib)
 set(EXECUTABLE_OUTPUT_PATH ${PROJECT_BINARY_DIR}/bin)
-set(CUSTOM_INCLUDE_DIR "" CACHE FILEPATH "Extra include path")
-set(CUSTOM_LINK_DIR "" CACHE FILEPATH "Extra library path")
 
 # place binaries for all build types in the same directory, so we know where to find it
 # when running tests or generating docs
@@ -123,14 +121,6 @@ else ()
 endif()
 set(LANG_CODES ${lcodes} CACHE STRING "List of language codes for which translations should be compiled in")
 
-if (CUSTOM_INCLUDE_DIR)
-   include_directories(${CUSTOM_INCLUDE_DIR})
-endif()
-
-if (CUSTOM_LINK_DIR)
-   link_directories(${CUSTOM_LINK_DIR})
-endif()
-
 if (win_static)
     set(CompilerFlags
         CMAKE_CXX_FLAGS