configure: produce useful uninstalled pc files.

The old files were scattered in several directories,
with an unusual name, but dependencies to the usual name,
and relative paths.

The new files are regrouped in a singled directory
(doc/examples/pc-uninstalled) with their usual name
and more absolute paths.

With this change, it becomes possible to compile the examples
with the ffmpeg build tree by setting the environment variable:
PKG_CONFIG_PATH=pc-uninstalled

configure

@@ -3958,18 +3958,22 @@ Libs: -L\${libdir} -l${shortname} $(enabled shared || echo $libs)
 Libs.private: $(enabled shared && echo $libs)
 Cflags: -I\${includedir}
 EOF
-cat <<EOF > $name/$name-uninstalled.pc
+
+mkdir -p doc/examples/pc-uninstalled
+includedir=${source_path}
+[ "$includedir" = . ] && includedir="\${pcfiledir}/../../.."
+cat <<EOF > doc/examples/pc-uninstalled/$name.pc
 prefix=
 exec_prefix=
-libdir=\${pcfiledir}
-includedir=${source_path}
+libdir=\${pcfiledir}/../../../$name
+includedir=${includedir}
 
 Name: $name
 Description: $comment
 Version: $version
 Requires: $requires
 Conflicts:
-Libs: \${libdir}/${LIBPREF}${shortname}${LIBSUF} $libs
+Libs: -L\${libdir} -l${shortname} $(enabled shared || echo $libs)
 Cflags: -I\${includedir}
 EOF
 }