Bug 759076 - Enable ccache for Linux 64 debug builds. r=ted,bhearsum

browser/config/mozconfigs/linux64/debug

@@ -13,6 +13,9 @@ export MOZILLA_OFFICIAL=1
 # Enable parallel compiling
 mk_add_options MOZ_MAKE_FLAGS="-j4"
 
+# Use ccache
+ac_add_options --with-ccache=/usr/bin/ccache
+
 # Treat warnings as errors in directories with FAIL_ON_WARNINGS.
 ac_add_options --enable-warnings-as-errors