graphics/delaboratory: fix build with GCC-based architectures

Respect LDFLAGS to fix build on GCC architectures:
usr/local/lib/libwebkitgtk-1.0.so.0: undefined reference to `vtable for std::cxx11::basic_ostringstream<char, std::char_traits<char>, std::allocator<char> >@GLIBCXX_3.4.21'
usr/local/lib/libwebkitgtk-1.0.so.0: undefined reference to `std::condition_variable::notify_one()@GLIBCXX_3.4.11'
//usr/local/lib/libwebkitgtk-1.0.so.0: undefined reference to `std::once_call@GLIBCXX_3.4.11'

While here, add USES=gnome.

PR: 237145
Approved by: portmaster@bsdforge.com (maintainer timeout), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20105