select.mk: Tune ccache options

With these adjustments, cached object files can now be shared across
multiple Android trees on a single workstation:

* Compiler binary modification time no longer causes cache misses,
  as a hash of the compiler binary is taken rather than looking
  at the mtime and size
* Header file modification time no longer causes cache misses,
  the headers are now always hashed
* Macros such as __DATE__ and __TIME__ no longer cause cache misses
* All paths in preprocessed source (due to use of -g) are now relative

These options require ccache 3.1 or later to be used; ccache 2.4 will
ignore these.

Change-Id: I7839637e9dacf7240bd9b7bfaa6406b1db423560
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/core/combo/select.mk b/core/combo/select.mk
index 803cfc0..e485d00 100644
--- a/core/combo/select.mk
+++ b/core/combo/select.mk
@@ -60,6 +60,23 @@
 include $(BUILD_COMBOS)/$(combo_target)$(combo_os_arch).mk
 
 ifneq ($(USE_CCACHE),)
+  # The default check uses size and modification time, causing false misses
+  # since the mtime depends when the repo was checked out
+  export CCACHE_COMPILERCHECK := content
+
+  # See man page, optimizations to get more cache hits
+  # implies that __DATE__ and __TIME__ are not critical for functionality.
+  # Ignore include file modification time since it will depend on when
+  # the repo was checked out
+  export CCACHE_SLOPPINESS := time_macros,include_file_mtime,file_macro
+
+  # Turn all preprocessor absolute paths into relative paths.
+  # Fixes absolute paths in preprocessed source due to use of -g.
+  # We don't really use system headers much so the rootdir is
+  # fine; ensures these paths are relative for all Android trees
+  # on a workstation.
+  export CCACHE_BASEDIR := /
+
   CCACHE_HOST_TAG := $(HOST_PREBUILT_TAG)
   # If we are cross-compiling Windows binaries on Linux
   # then use the linux ccache binary instead.