FreeBSD has a different GCC binary name

tools/build.py

     print(f'Using previously built DDS executable: {dds_exe}')
     if os.name == 'nt':
         tc_fpath = ROOT / 'tools/msvc.dds'
+    elif sys.platform.startswith('freebsd'):
+        tc_fpath = ROOT / 'tools/freebsd-gcc-9.dds'
     else:
         tc_fpath = ROOT / 'tools/gcc-9.dds'
 

tools/freebsd-gcc-9.dds

+Compiler-ID: GNU
+C++-Version: C++17
+C-Compiler: gcc9
+C++-Compiler: g++9
+# Range-v3 0.10.0 contains an accidental conversion warning
+Flags: -D SPDLOG_COMPILED_LIB -fconcepts -Werror=return-type -Wno-conversion
+Link-Flags: -static-libgcc -static-libstdc++
+Debug:  True
+Compiler-Launcher: ccache