pkg-config not finding x11

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I try to build libXrandr on OS X (actually I'm trying to build wireshark).
I get

checking for X... configure: error: Package requirements (x11) were not met:

No package 'x11' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables X_CFLAGS
and X_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.

and indeed pkg-config doesn't know about x11.

I just tried: I get the same error on NetBSD.