cmake: fix CMAKE_MODULE_PATH assignment using list(APPEND ...)

Using a list allow the user to specify additional search paths for Find
Modules.

Allegedly this might be useful when dealing with uncommon
build environments, e.g. when cross compiling

Author: dacav

CMakeLists.txt

@@ -19,7 +19,7 @@ include(CheckSymbolExists)
 include(CTest)
 include(GNUInstallDirs)
 
-set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
+list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
 
 set(DEFAULT_PAM_DIR ${CMAKE_INSTALL_FULL_LIBDIR})
 if (CMAKE_SYSTEM_NAME STREQUAL Linux)