By default, if ccache is found in PATH when regen-makefile.sh runs,
use cached compiled C++ object files during the build (or populate
the cache in case an object is not cached yet). Objects are cached
in ccache's default cache directory (~/.ccache) unless overriden by
the CCACHE_DIR environment variable.
The USE_CCACHE environment variable overrides auto-detection
in regen-makefile.sh and disables (USE_CCACHE=0) or enables
(USE_CCACHE=1) use of ccache.
Rebuilding the test suite from scratch with a populated cache
is an order of magnitude faster than doing an initial build.
ccache only speeds up C++ (.cc) to object file (.o) compilation.
Linking object (.o) files into shared object (.so) files is still
performed by the real compiler during every build.
Change-Id: Ibb538f602206535c06980f88191c1dabe3c4cd82
526895e7d4