use := instead of ?=

use ":=" instead of "?=",
because "?=" does not work if PKG_CONFIG_PATH exists and openrtm.pc or openhrp3.pc are not included in PKG_CONFIG_PATH

Currently, it seems not to work groovy+rosbuild environment.
If 
```
source /opt/ros/groovy/setup.bash
```
is executed, PKG_CONFIG_PATH is as follow:
```
/opt/ros/groovy/lib/pkgconfig
```