jpeg-turbo: now checks for pkg-config

jpeg-turbo: now checks for pkg-config

The jpeg-turbo configure script now contains PKG_PROG_PKG_CONFIG, even
though it doesn't so far use pkg-config to detect any dependencies (it does
install a .pc file now though):

checking pkg-config is at least version 0.9.0... ./configure: line 13540: \
  /home/peko/source/buildroot/output/host/usr/bin/pkg-config: No such file or directory

Depend on host-pkgconf for consistency.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>