Ignition Transport fails to build on Debian/Ubuntu with pkg-config 0.29.2

[stefanor]

Upgrading pkg-config from 0.29-6 to 0.29.2 has broken FindZeroMQ.cmake, I think.

Building ignition-transport 8.0.0 in Debian fails with this error:

CMake Error in src/CMakeLists.txt:
  Imported target "ZeroMQ::ZeroMQ" includes non-existent path

    "-isystem"

  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:

  * The path was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and references files it does not
  provide.

Presumably caused by:

$ pkg-config --cflags-only-I libzmq
-isystem /usr/include/mit-krb5 -I/usr/include/pgm-5.2

It used to output:

$ pkg-config --cflags-only-I libzmq
-I/usr/include/pgm-5.2

The relevant pkg-config change is: https://cgit.freedesktop.org/pkg-config/commit/?id=d5d80748936c1ddb2e32bc713f2ed51dc6f753ea

Full build log: https://launchpadlibrarian.net/478683366/buildlog_ubuntu-groovy-amd64.ignition-transport_8.0.0+dfsg-3_BUILDING.txt.gz
Debian Bug: https://bugs.debian.org/960081

[jspricke]

This is a cmake bug tracked here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=959064

[stefanor]

Aha. And fixed too.