Bugzilla Bug 144551

The Commit 1.16 to mjpegtools-1.8.0-r1.ebuild  broke the ebuild / the configure
script of mjpegtools to no longer correctly check for libpng and no longer
install /usr/bin/png2yuv.

The associated changelog entry is:
  30 Jul 2006; Diego Petten

The associated changelog entry is:
  30 Jul 2006; Diego Pettenò <flameeyes@gentoo.org>
  +files/mjpegtools-1.8.0-libquicktime.patch, mjpegtools-1.8.0-r1.ebuild:
  Add patch from bug #128619 when using libquicktime 0.9.9, thanks to Mario
  Fetka; also fix quicktime useflag, thanks to Eldad Zack (eldad), both in bug
  #128619.

for latest mjpegtools-1.8.0-r1.ebuild png2yuv is installed when USE="quicktime"
(regardless wheather quicktime is really installed or only fooled with
--nodeps).

When USE="-quicktime" png2yuv is not installed.

The problem is within this part of configure.ac:
---cut---
have_libquicktime=false
if test x$with_libquicktime != xno ; then
  PKG_CHECK_MODULES(LIBQUICKTIME, [libquicktime >= 0.9.4],
                    [have_libquicktime=true
                     AC_DEFINE(HAVE_LIBQUICKTIME, 1, [libquicktime >= 0.9.4
present])],
                    [have_libquicktime=false])
fi

dnl *********************************************************************
dnl Check for PNG library
dnl   (creates LIBPNG_CFLAGS, LIBPNG_LIBS)
dnl *********************************************************************
have_libpng=false
if test x$with_libpng != xno ; then
  PKG_CHECK_MODULES(LIBPNG, [libpng], [have_libpng=true], [have_libpng=false])
  if test x$have_libpng = xfalse ; then
    PKG_CHECK_MODULES(LIBPNG, [libpng12], [have_libpng=true],
[have_libpng=false])
  fi
fi
---cut---

The PKG_CHECK_MODULES-startement for quicktime is the first occurrence of
pkg-config related things in configure.ac.
Thus it leads to automagic inclusion of PKG_PROG_PKG_CONFIG-macro.
The problem is that in the created configure-script the expanded
PKG_PROG_PKG_CONFIG is not in global-scope but contained inside the statement:

---cut---
if test x$with_libquicktime != xno ; then
...
fi
---cut---

And that means it is not executed when --without-libquicktime is on
configure-command-line.

This is the reason that the last commit from flameeyes triggered the problem as
before that parameter was not there.

Fixed by adding PKG_PROG_PKG_CONFIG in global scope to configure.ac.
Also added use-flag png and dependency to libpng.