Add qmake feature and configure option to use ccache

Enabled via configure --ccache, or CONFIG += ccache in 3rd party
projects.

Ensures that we use the right sloppiness and other ccache options
during compilation.

Task-number: QTBUG-31034
Change-Id: I696b3d3f0398873a29b93d1bc2b4d4e06ef23dc9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

config_help.txt

@@ -144,6 +144,7 @@ Build options:
   -ltcg ................ Use Link Time Code Generation [no]
   -use-gold-linker ..... Use the GNU gold linker [auto]
   -incredibuild-xge .... Use the IncrediBuild XGE [no] (Windows only)
+  -ccache .............. Use the ccache compiler cache [no] (Unix only)
   -make-tool <tool> .... Use <tool> to build qmake [nmake] (Windows only)
   -mp .................. Use multiple processors for compilation (MSVC only)
 

configure.json

@@ -61,6 +61,7 @@
             "avx2": "boolean",
             "avx512": { "type": "boolean", "name": "avx512f" },
             "c++std": "cxxstd",
+            "ccache": { "type": "boolean", "name": "ccache" },
             "commercial": "void",
             "compile-examples": { "type": "boolean", "name": "compile_examples" },
             "confirm-license": "void",
@@ -620,6 +621,11 @@
             "label": "IncrediBuild",
             "type": "files",
             "files": [ "BuildConsole.exe", "xgConsole.exe" ]
+        },
+        "ccache": {
+            "label": "ccache",
+            "type": "files",
+            "files": [ "ccache" ]
         }
     },
 
@@ -1206,6 +1212,12 @@
             "condition": "tests.incredibuild_xge",
             "output": [ "publicConfig" ]
         },
+        "ccache": {
+            "label": "Using ccache",
+            "autoDetect": false,
+            "condition": "config.unix && tests.ccache",
+            "output": [ "privateConfig" ]
+        },
         "msvc_mp": {
             "label": "Use multiple processors when compiling with MSVC",
             "emitIf": "config.msvc",
@@ -1313,6 +1325,11 @@ Configure with '-qreal float' to create a build that is binary-compatible with 5
                     "type": "firstAvailableFeature",
                     "args": "c++1z c++14 c++11"
                 },
+                {
+                    "type": "feature",
+                    "args": "ccache",
+                    "condition": "config.unix"
+                },
                 "use_gold_linker",
                 {
                     "type": "feature",

mkspecs/features/unix/ccache.prf

@@ -0,0 +1,19 @@
+macx-xcode: return()
+
+darwin: load(sdk)
+
+ccache_prefix = ccache
+
+precompile_header {
+    CCACHE_SLOPPINESS += pch_defines time_macros
+
+    ccache_prefix = \
+        CCACHE_SLOPPINESS=\"$$join(CCACHE_SLOPPINESS, ',')\$${CCACHE_SLOPPINESS+,\$$CCACHE_SLOPPINESS}\" \
+        # Make sure we build sources directly, not from their preprocessed version,
+        # otherwise precompiled headers will not be used during cache misses.
+        CCACHE_CPP2=true \
+        $$ccache_prefix
+}
+
+for(tool, $$list(QMAKE_CC QMAKE_CXX QMAKE_LINK QMAKE_LINK_SHLIB QMAKE_LINK_C)): \
+    $$tool = $$ccache_prefix $$eval($$tool)