Without the environment variable C_INCLUDE_PATH set, and with X libs installed in a prefix specified with configure --prefix=, at-spi2-core's configure fails with:
checking for X... no
configure: error: X development libraries not found
With C_INCLUDE_PATH set to $installdir/include , it works.
I've heard C_INCLUDE_PATH should not be required. And I'm build testing lots of repos, and needing this is unusual.
Seen in git master commit 8b08de05a756edba6d40debdd9dce09cf0c58734 , which was recently the latest commit.
Git repo containing what it's referring to as "X development libraries" is git://anongit.freedesktop.org/xorg/lib/libX11
Debian package containing it is, I believe, libx11-dev.
Also, I'd prefer a pkg-config error that the package "x11" is missing, to make it easier to figure out what exactly it's looking for and not finding (instead of "X development libraries not found"). Or at least something more closely mimicking it.
The other environment variables I have set which I believe should be sufficient to find the X libs are:
ACLOCAL="aclocal -I $installdir/share/aclocal"