Fixing symbolic links

Soft links require an absolute path to work properly.  Before this change, `which /usr/local/bin/gcc` still resolves to `/usr/bin/gcc` even with `/usr/local/bin` being earlier in the `$PATH` variable.

Author: rplankenhorn

docs/build-speed.md

@@ -79,12 +79,12 @@ On Mac OS, we can install ccache with `brew install ccache`.
 Once installed you can configure it as follows to cache NDK compile results:
 
 ```
-ln -s ccache /usr/local/bin/gcc
-ln -s ccache /usr/local/bin/g++
-ln -s ccache /usr/local/bin/cc
-ln -s ccache /usr/local/bin/c++
-ln -s ccache /usr/local/bin/clang
-ln -s ccache /usr/local/bin/clang++
+ln -s $(which ccache) /usr/local/bin/gcc
+ln -s $(which ccache) /usr/local/bin/g++
+ln -s $(which ccache) /usr/local/bin/cc
+ln -s $(which ccache) /usr/local/bin/c++
+ln -s $(which ccache) /usr/local/bin/clang
+ln -s $(which ccache) /usr/local/bin/clang++
 ```
 
 This will create symbolic links to `ccache` inside the `/usr/local/bin/` which are called `gcc`, `g++`, and so on.