mkmf_config is more robust about detecting libdir

openbsd's pkg-config doesn't support `--print-variables`, but we don't
need it since everybody's pkg-config seems to just return a blank
string if the variable doesn't exist.

CHANGELOG.md

@@ -7,6 +7,7 @@
 - When setting the C compiler through the `MiniPortile` constructor, the preferred keyword argument is now `:cc_command`. The original `:gcc_command` is still supported. (#144 by @flavorjones)
 - GPG file verification error messages are captured in the raised exception. Previously these errors went to `stderr`. (#145 by @flavorjones)
 - Add support for extracting xz-compressed tarballs on OpenBSD. (#141 by @postmodern)
+- Add OpenBSD support to the experimental method `MakeMakefile#mkmf_config`. (#141 by @flavorjones)
 
 
 ### 2.8.6 / 2024-04-14

lib/mini_portile2/mini_portile.rb

@@ -320,10 +320,8 @@ def mkmf_config(pkg: nil, dir: nil, static: nil)
     if static
       libdir = lib_path
       if pcfile
-        variables = minimal_pkg_config(pcfile, "print-variables").split("\n").map(&:strip)
-        if variables.include?("libdir")
-          libdir = minimal_pkg_config(pcfile, "variable=libdir")
-        end
+        pcfile_libdir = minimal_pkg_config(pcfile, "variable=libdir").strip
+        libdir = pcfile_libdir unless pcfile_libdir.empty?
       end
 
       #