CMake: Use PROJECT_SOURCE_DIR to improve using mosquitto as subdirectory

When mosquitto is included as subdirectory, `CMAKE_SOURCE_DIR` does not refer to the mosquitto top level CMake file, but to the whole project top level CMake.
Use `PROJECT_SOURCE_DIR` instead to refer to the right CMake in both contextes.

Signed-off-by: Pierre Hallot <hallotpierre@gmail.com>

CMakeLists.txt

@@ -10,7 +10,7 @@ cmake_policy(SET CMP0042 NEW)
 project(mosquitto)
 set (VERSION 2.0.14)
 
-list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/")
+list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/")
 
 add_definitions (-DCMAKE -DVERSION=\"${VERSION}\")
 

man/CMakeLists.txt

@@ -8,10 +8,10 @@ if(NOT WIN32)
 	find_program(XSLTPROC xsltproc OPTIONAL)
 	if(XSLTPROC)
 		function(compile_manpage page)
-			add_custom_command(OUTPUT ${CMAKE_SOURCE_DIR}/man/${page}
-				COMMAND xsltproc ${CMAKE_SOURCE_DIR}/man/${page}.xml -o ${CMAKE_SOURCE_DIR}/man/
-				MAIN_DEPENDENCY ${CMAKE_SOURCE_DIR}/man/${page}.xml)
-			add_custom_target(${page} ALL DEPENDS ${CMAKE_SOURCE_DIR}/man/${page})
+			add_custom_command(OUTPUT ${PROJECT_SOURCE_DIR}/man/${page}
+				COMMAND xsltproc ${PROJECT_SOURCE_DIR}/man/${page}.xml -o ${PROJECT_SOURCE_DIR}/man/
+				MAIN_DEPENDENCY ${PROJECT_SOURCE_DIR}/man/${page}.xml)
+			add_custom_target(${page} ALL DEPENDS ${PROJECT_SOURCE_DIR}/man/${page})
 		endfunction()
 
 		compile_manpage("mosquitto_ctrl.1")