osx: link up ccache

This installs ccache from homebrew, activates it, and verfies its operation.

travis_osx_steps.sh

@@ -13,6 +13,16 @@ source $MULTIBUILD_DIR/library_builders.sh
 # config.sh can override any function defined here.
 
 function before_install {
+    if [ $USE_CCACHE == 1 ]; then
+        brew install ccache
+        export PATH=/usr/local/opt/ccache/libexec:$PATH
+        export CCACHE_MAXSIZE=200M
+        export CCACHE_CPP2=1
+        
+        # Prove to the developer that ccache is activated
+        which clang
+    fi
+
     export CC=clang
     export CXX=clang++
     get_macpython_environment $MB_PYTHON_VERSION venv