I have just tried building ModemManager-1.2-rc1 and saw the same error. (I needed to run the autogen.sh script on the extracted source first). Apparently it requires qmi-glib >= 1.6 which is later than the 1.4.0 in Slackware 14.1
You will need to upgrade the libqmi package. The source is
here
[EDIT]I have managed to build a package for ModemManager-1.2-rc1
I downloaded the source for libqmi-1.8.0 (libqmi-1.8.0.tar.gz) and extracted it with 'tar xvf libqmi-1.8.0.tar.gz'. I did 'cd ./libqmi-1.8.0' then './autogen.sh' then 'cd ..' then 'tar cvf libqmi-1.8.0.tar.xz ./libqmi-1.8.0' to build a source archive that would work with the SlackBuild from the 14.1 source.
I did similar steps for ModemManager-1.2-rc1 with the addition that I ran the SlackBuild with the command 'VERSION=1.2-rc1 sh ./ModemManager.SlackBuild'
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