OK, this is driving me nuts. I needed glib-2.28 to build one single package that won't build with later glib, so I eradicated 2.31 and built 2.28, then I built my dependant package, eradicated glib-2.28 and re-built 2.31. After this I went on to successfully build xfce.

However, many programs using glib are now fine at ./configure, but are failing make, and the only common element is glib. The errors are:

error: #error "Only <glib.h> can be included directly."

There's usually the attempted include shown, but it varies depending on the package I'm trying to build. I'm sure there must be a CFLAG or something that needs setting, but I can find nothing.

Does anyone have any ideas?

Try googling for

I've googled it every which way, and whilst I can find possible fixes for building specific programs, nothing that points to the underlying cause to enable a fix across the board. (No fixes for the programs I'm trying to compile, either).

Well this post seems to have something to say on the matter.

Seems like the problem is that ./configure adds "-DG_DISABLE_SINGLUE_INCLUDES" to CFLAGS of the application being compiled, while some other libs and headers aren't ready for that.
So, in those applications, try to remove this flag from CFLAGS, either in ./configure or elsewhere.
Dirty fix.

...Although, here it gives to understand that such error should appear starting from as early as version glib-2.17. So must make no difference between 2.28/2.31 it seems.

Therefore, from the looks of it, the problem must be the applications using this CFLAGS definition in ./configure and configure.h.

Well, I tried an alternative fix, which was to go through the sources as they fail in make, and replace the includes with #include <glib.h>, then reconfigure and re make. This has allowed certainly one test application (xchat) to successfully build (and works after testing), so for now I'll mark this solved.