[oe] libproxy glib.h build error

[hobbesc7 at gmail.com]

Has anyone else had the following build error with libproxy?

/work/yocto/poky/build/tmp/work/armv7a-vfp-neon-poky-linux-gnueabi/libproxy-0.4.
7-r4/libproxy-0.4.7/libproxy/modules/pxgsettings.cpp:26:18: fatal error:
glib.h:     No such file or directory

On Google some people suggest adding the output of:

pkg-config --cflags glib-2.0

to CFLAGS, but I don't know how to do this within my crosscompile oe
environment.