The pkg-config executable hardcodes these variables, the Makefile does not expand them like ports do and ports land is somewhat shifted to hardcode those paths anyway.
OK? Index: pkg-config.1 =================================================================== RCS file: /cvs/src/usr.bin/pkg-config/pkg-config.1,v retrieving revision 1.35 diff -u -p -r1.35 pkg-config.1 --- pkg-config.1 15 Sep 2020 07:18:45 -0000 1.35 +++ pkg-config.1 15 Dec 2021 18:38:58 -0000 @@ -43,13 +43,13 @@ Libraries from base install their metada packages controlled by the .Xr ports 7 system store their metadata in -.Pa ${LOCALBASE}/lib/pkgconfig +.Pa /usr/local/lib/pkgconfig and -.Pa ${LOCALBASE}/share/pkgconfig ; +.Pa /usr/local/share/pkgconfig ; the X Window System stores its metadata in -.Pa ${X11BASE}/lib/pkgconfig +.Pa /usr/X11R6/lib/pkgconfig and -.Pa ${X11BASE}/share/pkgconfig . +.Pa /usr/X11R6/share/pkgconfig . The .Nm utility will search these locations by default.