Revert "pkg-config file: remove exec_prefix, unused"
Revert "pkg-config file: remove exec_prefix, unused"

This reverts commit 28b7e23f680e23c02ea63b5c0654eba43cad3091.

I didn't know, but it's actually used. The generated *.pc file contains:

libdir=${exec_prefix}/lib64

This is not necessary with some other build systems.

GNOME/gspell,GNOME/gspell