How to retrieve prefix using pkg-config?

[Jeffrey Walton]

Hi Everyone,

I'm trying to build the GnuPG suite. The components do not honor
PKG_CONFIG_PATH so I need to do something like this:

if ! ./configure \
     --prefix="${PREFIX}" \
     --with-libgpg-error-prefix="$(pkg-config --prefix libgpg-error)";
then
    echo "Failed to configure libgcrypt"
    exit 1
fi

The code above is producing an error, "Unknown option --prefix". The
pkg-config(1) man page does not say how to extract the prefix to use.

How do I retrieve the prefix field from a *.pc file?