${pcfiledir} refers to the directory of the package-config file. For an
xrdp build in the source tree, the correct include location would be
${pcfiledir}/../common, not ${pcfiledir}/common
For xrdp build outside the source tree, there is no way to find the
source tree without help from the configure script.
While the '${pc_top_builddir}/${pcfiledir}' pattern is still widespread,
many packages (e.g. gstreamer) are switching to @abs_top_srcdir@ and
@abs_top_builddir@, which are always absolute and have knowledge about
the source tree location from the configure script.
Use @abs_top_srcdir@ for the headers. @abs_top_builddir@ is not needed,
as xrdp produces no externally consumable libraries or generated
headers.
