Disable build system pkg-config when cross-compiling to non-Linux

This restores a modified form of the original commit 893ebbab0c (Disable
use of build system pkg-config files when cross-compiling, 2020-12-13),
thus mostly reverting 6dbf59f34f (Revert "Disable use of build system
pkg-config files when cross-compiling", 2022-10-25) and now disables the
use of build system pkg-config files only when cross-compiling for
non-Linux hosts.

This seems like the best behaviour by default because it avoids
unexpected problems when cross-compiling for MSW or iOS where build
system .pc files should never be used, but still uses these files, if
they are available, when cross-compiling for other Linux systems.

This is Linux-specific currently but should probably be extended to
other Unix systems too. In any case, the default behaviour can always be
overridden by setting PKG_CONFIG_LIBDIR manually.

See #22886.

configure

@@ -25051,6 +25051,26 @@ $as_echo "no" >&6; }
 
 fi
 
+if test "$build" != "$host"; then
+    case "${host}" in
+        *-linux-*)
+            ;;
+
+        *)
+            disable_build_system_pkg_config=1
+            ;;
+    esac
+fi
+
+if test "$disable_build_system_pkg_config" = 1; then
+
+
+                if test -z "$PKG_CONFIG_LIBDIR"; then
+        PKG_CONFIG_LIBDIR=/dev/null
+        export PKG_CONFIG_LIBDIR
+    fi
+fi
+
 
 
 if test "$wxUSE_REGEX" != "no"; then