Improve precompiled header compatibility with (s)ccache

.gitignore

@@ -8,6 +8,7 @@ autom4te.cache
 *.pyc
 *.dSYM
 *.gch
+*.pch
 *.swp
 *~.nib
 *.pbxuser

build/cxx_tests.rb

@@ -158,7 +158,15 @@
 end
 
 let(:test_cxx_flags) do
-  ['-include test/cxx/TestSupport.h'] + basic_test_cxx_flags
+  # Some flags are necessary to make precompiled headers play well with ccache (and possibly also sccache):
+  # https://ccache.dev/manual/4.8.2.html#_precompiled_headers
+  if PlatformInfo.cxx_is_gcc?
+    ['-include test/cxx/TestSupport.h', '-fpch-preprocess']
+  elsif PlatformInfo.cxx_is_clang?
+    ["-include-pch test/cxx/TestSupport.h.#{PlatformInfo.precompiled_header_extension}", '-Xclang', '-fno-pch-timestamp']
+  else
+    ['-include test/cxx/TestSupport.h']
+  end + basic_test_cxx_flags
 end
 
 let(:test_cxx_ldflags) do
@@ -182,7 +190,7 @@
     lambda { {
       :include_paths => test_cxx_include_paths,
       :flags => test_cxx_flags,
-      :deps => 'test/cxx/TestSupport.h.gch'
+      :deps => "test/cxx/TestSupport.h.#{PlatformInfo.precompiled_header_extension}"
     } }
   )
 end
@@ -280,14 +288,15 @@
   end
 end
 
-file('test/cxx/TestSupport.h.gch' => generate_compilation_task_dependencies('test/cxx/TestSupport.h')) do
+file("test/cxx/TestSupport.h.#{PlatformInfo.precompiled_header_extension}" => generate_compilation_task_dependencies('test/cxx/TestSupport.h')) do
   compile_cxx(
-    'test/cxx/TestSupport.h.gch',
+    "test/cxx/TestSupport.h.#{PlatformInfo.precompiled_header_extension}",
     'test/cxx/TestSupport.h',
     :include_paths => test_cxx_include_paths,
     :flags => [
       "-x c++-header",
+      PlatformInfo.cxx_is_clang? ? "-Xclang -emit-pch" : nil,
       basic_test_cxx_flags
-    ].flatten
+    ].compact.flatten
   )
 end

build/test_basics.rb

@@ -34,7 +34,7 @@
 desc "Clean all compiled test files"
 task 'test:clean' do
   sh("rm -rf #{TEST_OUTPUT_DIR}")
-  sh("rm -f test/cxx/*.gch")
+  sh("rm -f test/cxx/*.#{PlatformInfo.precompiled_header_extension} test/cxx/*.gch test/cxx/*.pch")
 end
 
 task :clean => 'test:clean'

src/ruby_supportlib/phusion_passenger/platform_info/compiler.rb

@@ -603,6 +603,14 @@ def self.export_dynamic_flags
       end
     end
 
+    def self.precompiled_header_extension
+      if cxx_is_gcc?
+        'gch'
+      else
+        'pch'
+      end
+    end
+
 
     def self.make
       return string_env('MAKE', find_command('make'))