gnu: Move pkg-config to 'native-inputs' in various places.

* gnu/packages/glib.scm (libsigc++): Move pkg-config to
  'native-inputs'.
  (glibmm): Likewise.
* gnu/packages/mpd.scm (mpd): Likewise.
* gnu/packages/package-management.scm (guix-0.7): Likewise.
* gnu/packages/sdl.scm (sdl-net): Likewise.
  (sdl-ttf): Likewise.
* gnu/packages/xiph.scm (vorbis-tools): Likewise.

gnu/packages/glib.scm

@@ -354,8 +354,8 @@ by GDBus included in Glib.")
               (base32
                "14q3sq6d43f6wfcmwhw4v1aal4ba0h5x9v6wkxy2dnqznd95il37"))))
     (build-system gnu-build-system)
-    (inputs `(("pkg-config" ,pkg-config)))
-    (native-inputs `(("m4" ,m4)))
+    (native-inputs `(("pkg-config" ,pkg-config)
+                     ("m4" ,m4)))
     (home-page "http://libsigc.sourceforge.net/")
     (synopsis "Type-safe callback system for standard C++")
     (description
@@ -397,8 +397,8 @@ has an ease of use unmatched by other C++ callback libraries.")
                      (("Gio::init.*$")
                       "return 77;\n")))
                  %standard-phases)))
-    (inputs `(("pkg-config" ,pkg-config)
-              ("glib" ,glib "bin")))
+    (native-inputs `(("pkg-config" ,pkg-config)
+                     ("glib" ,glib "bin")))
     (propagated-inputs
      `(("libsigc++" ,libsigc++)
        ("glib" ,glib)))

gnu/packages/mpd.scm

@@ -94,10 +94,10 @@ interfacing MPD in the C, C++ & Objective C languages.")
               ("libsndfile" ,libsndfile)
               ("libvorbis" ,libvorbis)
               ("opus" ,opus)
-              ("pkg-config" ,pkg-config)
               ("pulseaudio" ,pulseaudio)
               ("sqlite" ,sqlite)
               ("zlib" ,zlib)))
+    (native-inputs `(("pkg-config" ,pkg-config)))
     ;; Missing optional inputs:
     ;;   libyajl
     ;;   libcdio_paranoia

gnu/packages/package-management.scm

@@ -75,6 +75,7 @@
                    (copy "mips64el")
                    #t)
                  %standard-phases)))
+    (native-inputs `(("pkg-config" ,pkg-config)))
     (inputs
      (let ((boot-guile (lambda (arch hash)
                          (origin
@@ -90,7 +91,6 @@
          ("sqlite" ,sqlite)
          ("libgcrypt" ,libgcrypt)
          ("guile" ,guile-2.0)
-         ("pkg-config" ,pkg-config)
 
          ("boot-guile/i686"
           ,(boot-guile "i686"

gnu/packages/sdl.scm

@@ -212,7 +212,7 @@ MIDI, Ogg Vorbis, and MP3.")
                 "1d5c9xqlf4s1c01gzv6cxmg0r621pq9kfgxcg3197xw4p25pljjz"))))
     (build-system gnu-build-system)
     (propagated-inputs `(("sdl" ,sdl)))
-    (inputs `(("pkg-config" ,pkg-config)))
+    (native-inputs `(("pkg-config" ,pkg-config)))
     (synopsis "SDL networking library")
     (description "SDL_net is a small, cross-platform networking library for
 SDL.")
@@ -234,8 +234,8 @@ SDL.")
     (build-system gnu-build-system)
     (propagated-inputs `(("sdl" ,sdl)))
     (inputs `(("freetype" ,font:freetype)
-              ("mesa" ,mesa)
-              ("pkg-config" ,pkg-config)))
+              ("mesa" ,mesa)))
+    (native-inputs `(("pkg-config" ,pkg-config)))
     (synopsis "SDL TrueType font library")
     (description "SDL_ttf is a TrueType font rendering library for SDL.")
     (home-page "http://www.libsdl.org/projects/SDL_ttf/")

gnu/packages/xiph.scm

@@ -271,8 +271,8 @@ Kate stream.")
              ("libkate" ,libkate)
              ("libogg" ,libogg)
              ("libvorbis" ,libvorbis)
-             ("pkg-config" ,pkg-config)
              ("speex" ,speex)))
+   (native-inputs `(("pkg-config" ,pkg-config)))
    (synopsis "Ogg vorbis tools")
    (description
     "Ogg vorbis is a non-proprietary, patent-and-royalty-free,