libxdamage depends on libx11 as you found out. Depending on how you install libx11, x11.pc will be in {SOMEPREFIX}/pkgconfig/x11.pc. Worst case you could
Code:
find / -name x11.pc
to find x11.pc. If you don't find it, you're gonna need to re-install libx11 (and pay attention for errors). If you do find it, then
Code:
export PKG_CONFIG_PATH=/where/ever/you/found/it/pkgconfig/x11.pc:$PKG_CONFIG_PATH
before you try libxdamage again should do it.