I miss several components, plus the pkgconfig provided by gecko-dev is unfortunately inconsistent
```
prefix=/usr
sdkdir=/usr/lib/xulrunner-qt5-devel-60.9.1
includedir=/usr/include/xulrunner-qt5-60.9.1
idldir=/usr/share/idl/xulrunner-qt5-60.9.1

Name: libxul
Description: The Mozilla Runtime and Embedding Engine
Version: 60.9.1
Requires: mozilla-nspr >=
Libs: -L${sdkdir}/lib -lxpcomglue -lxul -Wl,-rpath,/usr/lib/xulrunner-qt5-60.9.1
Cflags: -I${includedir} 
~                             
```