Using -D_POSIX_C_SOURCE on FreeBSD means POSIX 1003.1-1988 with pre-C89 environment, which hides va_copy in C99 and causes linking error:

[1/52] Compiling C object 'dbind/dbind@sta/dbind.c.o'.
../../source/at-spi2-core/dbind/dbind.c:58:3: warning: implicit declaration of function 'gettimeofday' is invalid in C99 [-Wimplicit-function-declaration]
  gettimeofday (&tv, NULL);
  ^
../../source/at-spi2-core/dbind/dbind.c:144:3: warning: implicit declaration of function 'va_copy' is invalid in C99 [-Wimplicit-function-declaration]
  va_copy (args_demarshal, args);
  ^
2 warnings generated.
[6/52] Linking target dbind/dbind-test.
FAILED: dbind/dbind-test
clang  -o dbind/dbind-test 'dbind/dbind-test@exe/dbtest.c.o' 'dbind/dbind-test@exe/.._atspi_atspi-gmain.c.o' -L/home/lantw44/gnome/devinstall/lib -L/usr/local/lib -Wl,--no-undefined -Wl,--as-needed -march=corei7 -B/home/lantw44/.local/bin -pipe -g3 -O0 -Wl,--start-group dbind/libdbind.a -L/usr/local/lib -ldbus-1 -L/home/lantw44/gnome/devinstall/lib -lglib-2.0 -lintl -Wl,--end-group -L/usr/local/lib -ldbus-1 -L/home/lantw44/gnome/devinstall/lib -lglib-2.0 -lintl
dbind/libdbind.a(dbind.c.o): In function `dbind_method_call_reentrant_va':
/home/lantw44/gnome/build/at-spi2-core/../../source/at-spi2-core/dbind/dbind.c:144: undefined reference to `va_copy'
clang: error: linker command failed with exit code 1 (use -v to see invocation)