Install the .pc files to staging directory to help other packages
to find the libraries.
Since the build does not use CMake, we need to manually install the
files and replace two variables using sed.
Filed upstream as https://github.com/eclipse/mosquitto/pull/950
Signed-off-by: Michael Heimpold <michael.heimpold@i2se.com>
Tested-by: Karl Palsson <karlp@etactica.com>