Pkg_config_path

[Oblivion_4]

I was trying to answer some guys question earlier about compiling a driver from source and the compiler wasn't finding a certain library. I was going to recommend to the user to alter their PKG_CONFIG_PATH variable to be able to find the library but when I ran the command echo $PKG_CONFIG_PATH on my own machine I didn't receive any output.

So I'm wondering how does the compiler find libraries that are installed in ubuntu without a PKG_CONFIG_PATH variable set to /usr/include/pkgconfig?