In the mean time, Heikki Junnila added the necessary stuff in the trunk. Thanks to his changes, I can now build the RPM out of the latest SVN revision without any hack.

You can find a RPM of the revision #1235 in my repository.

About the compilation of the FTDI output plugin, here is the error message I have (with latest QLC trunk, aka SVN revision #1179):

(...)
g++ -Wl,--as-needed -Wl,--no-undefined -Wl,-z,relro -Wl,-O1 -shared -o libftdidmxout.so configureftdidmx.o ftdidmx.o ftdidmxdevice.o moc_configureftdidmx.o moc_ftdidmx.o moc_ftdidmxdevice.o   -L/usr/lib64 libftd2xx.a.0.4.16 -lQtGui -L/usr/lib64 -L/usr/lib -pthread -lpng -lfreetype -lgobject-2.0 -lSM -lICE -pthread -pthread -lXrender -lfontconfig -lXext -lX11 -lQtCore -lz -lm -pthread -lgthread-2.0 -lrt -lglib-2.0 -ldl -lpthread
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(ftd2xx.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(Event.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(usb_control.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(bulk_in.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(ftee2232.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(ftee232.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(ftee232r.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(serialno.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(ftee2232h.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(ftee4232h.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(usb.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(linux.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(error.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(descriptors.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(baud.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(fte2data.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `libftd2xx.a.0.4.16(eedata.o)' is incompatible with i386:x86-64 output
/usr/bin/ld: Dwarf Error: Offset (692) greater than or equal to .debug_str size (165).
libftd2xx.a.0.4.16(serialno.o): In function `i64toa':
/home/amiller/Desktop/Linux_D2XX/eeprom/serialno.c:48: undefined reference to `__moddi3'
/home/amiller/Desktop/Linux_D2XX/eeprom/serialno.c:49: undefined reference to `__divdi3'
/usr/bin/ld: final link failed: No space left on device
collect2: ld a retourné 1 code d'état d'exécution
make[2]: *** [libftdidmxout.so] Erreur 1
make[2]: quittant le répertoire « /home/maison/rpm/BUILD/qlc-3.0.0.trunk.r1179/libs/ftdidmx »
make[1]: *** [sub-ftdidmx-make_default] Erreur 2
make[1]: quittant le répertoire « /home/maison/rpm/BUILD/qlc-3.0.0.trunk.r1179/libs »
make: *** [sub-libs-make_default] Erreur 2
make: *** Attente des tâches non terminées....
(...)

My dirty fix was to disable the FTDI plugin compilation with this one-liner just before running qmake:

sed -i -e 's,SUBDIRS\t\t\t+= ftdidmx,#SUBDIRS\t\t\t+= ftdidmx,g' libs/libs.pro