qtopia4: fix use of pkg-config

When cross compiling, Qt sets PKG_CONFIG in its own configure script to
an empty string. That leads to not using pkg-config. To avoid that, use
the configure option -force-pkg-config. Also set environment variables
for pkg-config defined in TARGET_CONFIGURE_OPTS.

Patch by Markus Heidelberg.

See http://buildroot.uclibc.org/lists/buildroot/2008-October/011324.html
for more details.