define-prefix: skip one level if it ends up in /lib

In Debian and Ubuntu the multiarch filesystem layout looks as follows:

/usr/lib/x86_64-linux-gnu/pkgconfig/
/usr/lib/i386-linux-gnu/pkgconfig/

Which means when --define-prefix skips to the grand-parent
directory of pkgconfig, it ends up in /usr/lib rather than
/usr as it's supposed to.

This causes for example the include path to be printed as:
/usr/lib/include
which is wrong and breaks applications.

Check if the new directory is 'lib' and if it is go one step
above. A new series of unit tests is added with this layout.

check/check-relocatable

@@ -51,3 +51,22 @@ done
 RESULT="-I/some/path/include -L/some/path/lib -lfoo"
 run_test --define-prefix --cflags --libs empty-prefix
 run_test --dont-define-prefix --cflags --libs empty-prefix
+
+# Test prefix redefinition for .pc files when the pkgconfig directory is in
+# ..../lib/arch/ as Debian and Ubuntu and their derivatives do.
+PKG_CONFIG_LIBDIR="${abs_srcdir}/lib/arch/pkgconfig"
+for pkg in prefixdef prefixdef-expanded; do
+    # Typical redefinition
+    RESULT="-I${abs_top_srcdir}/check/include -L${abs_top_srcdir}/check/lib -lfoo"
+    run_test --define-prefix --cflags --libs $pkg
+
+    RESULT="-I/reloc/include -L/reloc/lib -lfoo"
+    run_test --dont-define-prefix --cflags --libs $pkg
+
+    # Non-standard redefinition
+    RESULT="-I/reloc/include -L${abs_top_srcdir}/check -lfoo"
+    run_test --define-prefix --prefix-variable=libdir --cflags --libs $pkg
+
+    RESULT="-I/reloc/include -L/reloc/lib -lfoo"
+    run_test --dont-define-prefix --cflags --libs $pkg
+done

check/lib/arch/pkgconfig

@@ -0,0 +1 @@
+../../pkgconfig

parse.c

@@ -1021,6 +1021,14 @@ parse_line (Package *pkg, const char *untrimmed, const char *path,
               q = g_path_get_dirname (pkg->pcfiledir);
               prefix = g_path_get_dirname (q);
               g_free (q);
+
+              /* Debian/Ubuntu use ..../lib/x86_64-linux-gnu/pkg-config */
+              if (g_str_has_suffix (prefix, "/lib"))
+                {
+                  q = prefix;
+                  prefix = g_path_get_dirname (q);
+                  g_free (q);
+                }
 	      
 	      /* Turn backslashes into slashes or
 	       * g_shell_parse_argv() will eat them when ${prefix}