Fixes broken Travis clang++ workaround.

Commit 66df930 disabled C++ builds on Travis when using clang++, in
order to get around an unresolved issue with __float128.  Hovever,
commit 2f6e5b6 broke the workround by adding an explicit qt=yes to
SCONSOPTS.  This wasn't immediately noticed, since that commit also
broke the use of Qt4 altogether, and there is no case that builds with
Qt5 and clang++.  Fixing the Qt4 bug in commit 2c01c13 then
reintroduced the old clang++ bug.

This change removes the troublesome explicit qt=yes.

TESTED:
Will be once this change is pushed and picked up by Travis.  It has no
effect outside Travis.

.travis/build.sh

@@ -17,7 +17,7 @@ SCONSOPTS="${SCONSOPTS} libdir=/usr/lib/${DEB_HOST_MULTIARCH}"
 SCONSOPTS="${SCONSOPTS} gpsd_user=gpsd"
 SCONSOPTS="${SCONSOPTS} gpsd_group=dialout"
 SCONSOPTS="${SCONSOPTS} debug=yes"
-SCONSOPTS="${SCONSOPTS} qt=yes"
+### SCONSOPTS="${SCONSOPTS} qt=yes"  # The default qt=yes must be overridable
 
 if dpkg -s qtbase5-dev 1>/dev/null 2>&1; then
     SCONSOPTS="${SCONSOPTS} qt_versioned=5"