Install ccache from Arm GCC.md

Install ccache for Arm GCC

1. Download GNU Arm Embedded Toolchain
2. Unpack: tar xf arm-none-eabi-gcc-<version>.tar.bz2
3. Edit your .bashrc or .bash_profile or similar:
   1. Add to PATH as a LAST member: export PATH="$PATH:$HOME/gcc-arm-none-eabi-9-2019-q4-major/bin"
   2. Create ~/bin and add to PATH as a first member: PATH=$HOME/bin:$PATH
4. Install ccache: sudo apt install ccache or brew install ccache
5. Add links to $HOME/bin:

cd ~/bin
for tool in gcc g++ cpp c++;do ln -s $(which ccache) arm-none-eabi-$tool;done

Mbed developer

Do not add GCC_ARM path into mbed config, this will skip the PATH end effectively skip the ccache.

Add following to your .bashrc:

export MBED_BUILD_TIMESTAMP=hardcoded_value_to_enable_ccache

This allows ccache to kick in on the preprocessor phase as well, because timestamps do not change anymore.