x11-wm/spectrwm: fix build on GCC architectures

Switch to newer GCC to workaround -Werror:
/usr/local/include/xcb/xinput.h:3072: warning: this decimal constant is unsigned only in ISO C90

Adding -Wno-error doesn't fix it, it's still overwritten by -Werror.

MFH: 2020Q3 (fix build blanket)