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.