net/guacamole-server: fix build on GCC architectures

Instead of removing -Werror from many Makefiles, just use newer GCC, which also works:
cc1: warnings being treated as errors
color-scheme.c: In function 'guac_terminal_parse_color_scheme':
color-scheme.c:262: warning: passing argument 4 of 'guac_terminal_parse_color_scheme_value' from incompatible pointer type