Those of you who have used qmake to build projects for any length of time on Linux will have probably come across the PKGCONFIG feature by now.

For those of you who haven't, here's a quick recap:
CONFIG += link_pkgconfig # enable the PKGCONFIG feature
PKGCONFIG += glib-2.0 # link against glib-2.0
While this looks simple, it has two small problems which aren't all that fun.

Firstly, it had what was really quite easily arguably a bug in that it didn't halt the qmake process if a requested package didn't exist. It would happily tell you it didn't exist, and then continue as if nothing had gone wrong, a very annoying problem in large projects:
$ cat
CONFIG += link_pkgconfig
PKGCONFIG += foobar

$ qmake
Package foobar was not found in the pkg-config search path.
Perhaps you should add the directory containing `foobar.pc'
to the PKG_CONFIG_PATH environment variable
No package 'foobar' found

$ echo $?

Secondly, it wasn't possible to (easily) check for optional dependencies and enable/disable functionality depending on the result of that check.

This has now changed! Thanks to Oswald for merging merge request 1022 into Qt, PKGCONFIG will, in the future, stop on error, and (even more fun) - we can now use the shiny new qmake packagesExist() test function to detect whether a library is installed:
CONFIG += link_pkgconfig

packagesExist(glib-2.0) {
    PKGCONFIG += glib-2.0

// and in the code:
#ifdef HAS_GLIB
    // use glib here
Much thanks to Marco for prompting me to finally try fix this (I've eternally been annoyed by it, but someone else being frustrated by it made me leap into action), as well as to Oswald, Marius and Arvid for their review and comments during writing, and finally to Murray and Mathias for respectively reporting and trying to patch the first issue in this post, a fact I only became aware of *after* finishing my patch. Great minds think alike, it seems!