build: re-enable PCH with CCache

Includes:
 - disable PCH validation when using XCode
 - disable Clang's PCH timestamping
 - add ostream header to the PCH

CMakeLists.txt

@@ -4,15 +4,15 @@
 cmake_minimum_required(VERSION 3.10..3.10)
 
 # Use ccache if available
-mark_as_advanced(CCACHE_FOUND)
+mark_as_advanced(CCACHE)
 find_program(CCACHE ccache)
 if(NOT ("${CCACHE}" STREQUAL "CCACHE-NOTFOUND"))
   set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
-  message(
-    STATUS "CCache is enabled. Precompiled headers are disabled by default."
-  )
-  set(NS3_PRECOMPILE_HEADERS OFF
-      CACHE BOOL "Precompile module headers to speed up compilation" FORCE
+  message(STATUS "CCache is enabled.")
+  execute_process(
+    COMMAND
+      ${CCACHE} --set-config
+      sloppiness=pch_defines,time_macros,include_file_mtime,include_file_ctime
   )
 endif()
 

build-support/macros-and-definitions.cmake

@@ -1236,6 +1236,17 @@ macro(process_options)
   endif()
 
   if(${PRECOMPILE_HEADERS_ENABLED})
+    if(CLANG)
+      # Clang adds a timestamp to the PCH, which prevents ccache from working
+      # correctly
+      # https://github.com/ccache/ccache/issues/539#issuecomment-664198545
+      add_definitions(-Xclang -fno-pch-timestamp)
+    endif()
+    if(${XCODE})
+      # XCode is weird and messes up with the PCH, requiring this flag
+      # https://github.com/ccache/ccache/issues/156
+      add_definitions(-Xclang -fno-validate-pch)
+    endif()
     set(precompiled_header_libraries
         <algorithm>
         <cstdlib>
@@ -1247,6 +1258,7 @@ macro(process_options)
         <list>
         <map>
         <math.h>
+        <ostream>
         <set>
         <sstream>
         <stdint.h>