Build fails because of signed/unsigned compare warning

I'm trying to build qt like this:

./configure -confirm-license -opensource
-prefix /usr
-bindir /usr/lib/qt/bin
-docdir /usr/share/doc/qt
-headerdir /usr/include/qt
-archdatadir /usr/lib/qt
-datadir /usr/share/qt
-sysconfdir /etc/xdg
-examplesdir /usr/share/doc/qt/examples
plugin-sql

-system-sqlite
-openssl-linked
-nomake examples
-no-rpath
-optimized-qmake
-dbus-linked
-system-harfbuzz
-journald
-reduce-relocations ${SSE2}
-developer-build

make -j4

The compiler gives the following warning and aborts because of the -Werror compiler flag:

../gui/text/qfontengine_ft.cpp:1126:39: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
for (int x = 0; x < slot->bitmap.width; x++) {

../gui/text/qfontengine_ft.cpp:1137:39: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
for (int x = 0; x < slot->bitmap.width; x++) {

../gui/text/qfontengine_ft.cpp:1146:39: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
for (int x = 0; x < slot->bitmap.width; x++) {

The output of the configure and make command is attached.