Initialize CMAKE_EXPORT_COMPILE_COMMANDS only if not set already

Also de-duplicate its initialization code.

Fixes: #16588

Modules/CMakeGenericSystem.cmake

@@ -50,15 +50,9 @@ if(CMAKE_GENERATOR MATCHES "Make")
   if(DEFINED CMAKE_TARGET_MESSAGES)
     set_property(GLOBAL PROPERTY TARGET_MESSAGES ${CMAKE_TARGET_MESSAGES})
   endif()
-  if(CMAKE_GENERATOR MATCHES "Unix Makefiles")
-    set(CMAKE_EXPORT_COMPILE_COMMANDS "$ENV{CMAKE_EXPORT_COMPILE_COMMANDS}"
-      CACHE BOOL "Enable/Disable output of compile commands during generation."
-      )
-    mark_as_advanced(CMAKE_EXPORT_COMPILE_COMMANDS)
-  endif()
 endif()
 
-if(CMAKE_GENERATOR MATCHES "Ninja")
+if(NOT DEFINED CMAKE_EXPORT_COMPILE_COMMANDS AND CMAKE_GENERATOR MATCHES "Ninja|Unix Makefiles")
   set(CMAKE_EXPORT_COMPILE_COMMANDS "$ENV{CMAKE_EXPORT_COMPILE_COMMANDS}"
     CACHE BOOL "Enable/Disable output of compile commands during generation."
     )

Tests/RunCMake/CMakeLists.txt

@@ -197,6 +197,9 @@ endif()
 add_RunCMake_test(CompilerNotFound)
 add_RunCMake_test(Configure -DMSVC_IDE=${MSVC_IDE})
 add_RunCMake_test(DisallowedCommands)
+if("${CMAKE_GENERATOR}" MATCHES "Unix Makefiles|Ninja")
+  add_RunCMake_test(ExportCompileCommands)
+endif()
 add_RunCMake_test(ExternalData)
 add_RunCMake_test(FeatureSummary)
 add_RunCMake_test(FPHSA)

Tests/RunCMake/ExportCompileCommands/BeforeProject-check.cmake

@@ -0,0 +1,4 @@
+if(NOT EXISTS "${RunCMake_TEST_BINARY_DIR}/compile_commands.json")
+  set(RunCMake_TEST_FAILED "compile_commands.json not generated")
+  return()
+endif()

Tests/RunCMake/ExportCompileCommands/BeforeProject.cmake

@@ -0,0 +1,3 @@
+enable_language(C)
+add_library(empty STATIC empty.c)
+message(STATUS "CMAKE_EXPORT_COMPILE_COMMANDS='${CMAKE_EXPORT_COMPILE_COMMANDS}'")

Tests/RunCMake/ExportCompileCommands/BeforeProjectBEFORE.cmake

@@ -0,0 +1 @@
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

Tests/RunCMake/ExportCompileCommands/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.17)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/ExportCompileCommands/RunCMakeTest.cmake

@@ -0,0 +1,3 @@
+include(RunCMake)
+
+run_cmake_with_options(BeforeProject -DCMAKE_PROJECT_INCLUDE_BEFORE=BeforeProjectBEFORE.cmake)