src/Makefile.am sets PKG_CONFIG_PATH to $(top_builddir) but it does this
without preserving the content of that environment variable, making it
fail in jhbuild with an error such as:

  GEN    GtkVnc-2.0.gir
  Package gtk+-3.0 was not found in the pkg-config search path.
  Perhaps you should add the directory containing `gtk+-3.0.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'gtk+-3.0' found
  Package gtk+-3.0 was not found in the pkg-config search path.
  Perhaps you should add the directory containing `gtk+-3.0.pc'
  to the PKG_CONFIG_PATH environment variable
  Package 'gtk+-3.0', required by 'GTK-VNC', not found
  In file included from <stdin>:4:0:
  /scratch/jhbuild/src/gtk-vnc-0.4.4/src/vnccairoframebuffer.h:25:21: fatal error: gdk/gdk.h: No such file or directory
  compilation terminated.