gtk on GNU/Linux: https is not supported

Dear list,

I am building netsurf 3.0 on mips64el-unknown-linux-gnu with:

sed -i '/LDFLAGS/s|lm|lm -ldl|' gtk/Makefile.target

make \
PREFIX=/usr \
TARGET=gtk

make install \
PREFIX=/usr \
TARGET=gtk

The build completes successfully.

Although OpenSSL 1.0.1e and GnuTLS 3.1.5 are installed, I have a pop-up window
saying "URIOpenError Operation not supported" when I try to open an https URI
with /usr/bin/netsurf.

What can be the problem?

Thanks,

Christophe