When you get the source from SVN you have all the development stuff so you don't need anything else.
If you did the usual configure; make; sudo make install it should have installed the geany.pc file for you, were there any errors in the process?
I just re-ran make install, the very last thing in install is to put geany.pc in $prefix/lib/pkgconfig so if you had any errors it might have missed it.
geany.pc was installed properly, just my pkg-config isn't smart enough to look into /usr/local/lib/pkgconfig and I had to set up PKG_CONFIG_PATH.
I managed to get my plugin compiled and it displays yellow underline for warnings :-) However, this yellow seems a little to bright. Do you have any preferences to what RGB should be used for that? Shouldn't API version be increased, since new feature is exposed?