@eigengrau I just managed to install it on a fresh version of the system, maybe you need to reinstall Qt4?

This currently fails to build with:

=== configuring in qucs (/home/seb/tmp/build/qucs/src/qucs-0.0.19/qucs)
configure: running /bin/sh ./configure --disable-option-checking '--prefix=/usr'  '--disable-doc' 'CXXFLAGS=-march=native -mtune=native -O2 -pipe -fstack-protector-strong -fno-plt -g -fvar-tracking-assignments -fdebug-prefix-map=/home/seb/tmp/build/qucs/src=/usr/src/debug' 'LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now' 'CPPFLAGS=-D_FORTIFY_SOURCE=2' 'CFLAGS=-march=native -mtune=native -O2 -pipe -fstack-protector-strong -fno-plt -g -fvar-tracking-assignments -fdebug-prefix-map=/home/seb/tmp/build/qucs/src=/usr/src/debug' --cache-file=/dev/null --srcdir=

…

checking for Qt headers... found in /usr/include/qt4 -I/usr/include/qt4/Qt -I/usr/include/qt4/QtGui -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtSvg -I/usr/include/qt4/QtXml -I/usr/include/qt4/QtScript -I/usr/include/qt4/Qt3Support
checking for Qt... 4 (multi-threaded)
checking for Qt library... configure: error: not found
configure: error: ./configure failed for qucs

@altazar: I merged your fixes :-)