build: fix configure when PKG_CONFIG is set
build: fix configure when PKG_CONFIG is set

If you define the environment variable PKG_CONFIG and assign it to
pkgconf (or pkg-config), configure will say it's OK, but it won't be
used.

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso