Drop USE_GCC and add missing build dependency

Backport an upstream commit that fixes the build with clang, which allows us to
drop USE_GCC. It is broken on at least FreeBSD 10:

===>  Configuring for flameshot-0.5.0
Project ERROR: Cannot run compiler 'g++'. Output:
===================
g++: not found
===================
Maybe you forgot to setup the environment?
*** Error code 3

Also add a dependency on qt5-buildtools, otherwise the build complains that rcc
is not found.

Approved by: portmgr (antoine)
Differential Revision: https://reviews.freebsd.org/D14100