Handle compile commands with shell callouts.

[dschaefer]

Looking at an example on Linux that uses pkg-config. The command line is:

gcc -std=c99 -Wall -Werror -o Release/cefcapi-gtk3 -I. -I.. -Wl,-rpath,. -L./Release examples/main_linux.c -lX11 -lcef pkg-config --libs --cflags gtk+-3.0

We need to expand what pkg-config returns before putting it in the compile_commands.json file. (Or do we? Make sure this works anyway).

An easy way to do that is to stick 'echo ' in front of it and send it to the shell for execution.