respect CMAKE_MODULE_PATH provided by user

Cross-compiling to Windows with MXE [1] requires providing custom
CMAKE_MODULE_PATH variable for CMake:

$ cmake -DCMAKE_MODULE_PATH=/file/provided/by/mxe.cmake

As Wesnoth's CMakeLists.txt resets CMAKE_MODULE_PATH, the effect of
CMAKE_MODULE_PATH provided by MXE is disabled and the build fails
with linking errors [2].

This commit changes CMakeLists.txt to respect the value of
user provided CMAKE_MODULE_PATH, so both MXE and Wesnoth
customizations of CMake variables are applied.

[1] http://mxe.cc/
[2] https://gist.github.com/starius/abfb9033d92f21ef521d

CMakeLists.txt

@@ -18,7 +18,7 @@ endif(COMMAND cmake_policy)
 set(CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS true)
 
 # use our own version of FindBoost.cmake and other Find* scripts
-set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake)
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
 
 #
 # Options
@@ -723,7 +723,7 @@ install(FILES l10n-track DESTINATION ${DATADIR})
 #
 
 configure_file(
-	"${CMAKE_MODULE_PATH}/uninstall.cmake.in"
+	"${CMAKE_SOURCE_DIR}/cmake/uninstall.cmake.in"
 	"${CMAKE_CURRENT_BINARY_DIR}/uninstall.cmake"
 	IMMEDIATE @ONLY
 )

doc/man/CMakeLists.txt

@@ -23,7 +23,7 @@ if(ENABLE_POT_UPDATE_TARGET)
 		COMMAND ${CMAKE_COMMAND}
 			-DPROJECT_SOURCE_DIR="${PROJECT_SOURCE_DIR}"
 			-DMANPAGES="${ALL_MANPAGES}"
-			-P "${CMAKE_MODULE_PATH}/po4a-man.cmake"
+			-P "${CMAKE_SOURCE_DIR}/cmake/po4a-man.cmake"
 		DEPENDS
 			${CMAKE_CURRENT_SOURCE_DIR}/wesnoth.6
 			${CMAKE_CURRENT_SOURCE_DIR}/wesnothd.6

doc/manual/CMakeLists.txt

@@ -83,7 +83,7 @@ if(ENABLE_POT_UPDATE_TARGET)
 			COMMAND ${CMAKE_COMMAND}
 				-DSOURCE="manual.${LINGUA}.xml"
 				-DCMD="${CMD}"
-				-P "${CMAKE_MODULE_PATH}/po4a-manual.cmake"
+				-P "${CMAKE_SOURCE_DIR}/cmake/po4a-manual.cmake"
 			COMMAND ${CMAKE_COMMAND} -E remove manual.${LINGUA}.xml
 			DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/manual.${LINGUA}.xml
 			COMMENT "[update-po4a-manual ${LINGUA}] Building ${LINGUA}.html."