CMake: Do not unconditionally override settings without good reason

fixes #342

CMakeLists.txt

@@ -11,7 +11,7 @@
 cmake_minimum_required(VERSION 3.15 FATAL_ERROR)
 cmake_policy(VERSION 3.15...3.20)
 
-set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake)
+list(PREPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake)
 include(GitHelper)
 get_git_version()
 

cmake/FairMQProjectSettings.cmake

@@ -25,7 +25,9 @@ set(PROJECT_MIN_CXX_STANDARD 17)
 if(CMAKE_CXX_STANDARD AND CMAKE_CXX_STANDARD VERSION_LESS PROJECT_MIN_CXX_STANDARD)
   message(FATAL_ERROR "A minimum CMAKE_CXX_STANDARD of ${PROJECT_MIN_CXX_STANDARD} is required.")
 endif()
-set(CMAKE_CXX_EXTENSIONS OFF)
+if(NOT DEFINED CMAKE_CXX_EXTENSIONS)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+endif()
 
 if(NOT BUILD_SHARED_LIBS)
   set(BUILD_SHARED_LIBS ON CACHE BOOL "Whether to build shared libraries or static archives")
@@ -37,7 +39,9 @@ if(NOT CMAKE_POSITION_INDEPENDENT_CODE)
 endif()
 
 # Generate compile_commands.json file (https://clang.llvm.org/docs/JSONCompilationDatabase.html)
-set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+if(NOT DEFINED CMAKE_EXPORT_COMPILE_COMMANDS)
+  set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+endif()
 
 # Define install dirs
 set(PROJECT_INSTALL_BINDIR ${CMAKE_INSTALL_BINDIR})
@@ -47,17 +51,19 @@ set(PROJECT_INSTALL_DATADIR ${CMAKE_INSTALL_DATADIR}/${PROJECT_NAME_LOWER})
 set(PROJECT_INSTALL_CMAKEMODDIR ${PROJECT_INSTALL_DATADIR}/cmake)
 
 # https://cmake.org/Wiki/CMake_RPATH_handling
-set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
+if(NOT DEFINED CMAKE_INSTALL_RPATH_USE_LINK_PATH)
+  set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
+endif()
 list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/${PROJECT_INSTALL_LIBDIR}" isSystemDir)
 if("${isSystemDir}" STREQUAL "-1")
   if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
-    set(CMAKE_EXE_LINKER_FLAGS ${CMAKE_EXE_LINKER_FLAGS} "-Wl,--enable-new-dtags")
-    set(CMAKE_SHARED_LINKER_FLAGS ${CMAKE_SHARED_LINKER_FLAGS} "-Wl,--enable-new-dtags")
-    set(CMAKE_INSTALL_RPATH "$ORIGIN/../${PROJECT_INSTALL_LIBDIR}")
+    list(APPEND CMAKE_EXE_LINKER_FLAGS "-Wl,--enable-new-dtags")
+    list(APPEND CMAKE_SHARED_LINKER_FLAGS "-Wl,--enable-new-dtags")
+    list(PREPEND CMAKE_INSTALL_RPATH "$ORIGIN/../${PROJECT_INSTALL_LIBDIR}")
   elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
-    set(CMAKE_INSTALL_RPATH "@loader_path/../${PROJECT_INSTALL_LIBDIR}")
+    list(PREPEND CMAKE_INSTALL_RPATH "@loader_path/../${PROJECT_INSTALL_LIBDIR}")
   else()
-    set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${PROJECT_INSTALL_LIBDIR}")
+    list(PREPEND CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/${PROJECT_INSTALL_LIBDIR}")
   endif()
 endif()
 
@@ -85,7 +91,7 @@ if(CMAKE_GENERATOR STREQUAL "Ninja" AND
   list(APPEND CMAKE_CXX_FLAGS "-fdiagnostics-color=always")
 endif()
 
-if(NOT PROJECT_VERSION_TWEAK)
+if(NOT DEFINED PROJECT_VERSION_TWEAK)
   set(PROJECT_VERSION_HOTFIX 0)
 else()
   set(PROJECT_VERSION_HOTFIX ${PROJECT_VERSION_TWEAK})