transmission: define _POSIX_C_SOURCE to allow realpath(foo, NULL)

Android 7 allows realpath() to be called with the resolved parameter as NULL.
(See tags/android-7.0.0_r1/libc/upstream-freebsd/lib/libc/stdlib/realpath.c#68)
In tr_sys_path_resolve(), a tiny bit of time can be saved by doing so.

packages/transmission/build.sh

@@ -35,6 +35,7 @@ termux_step_extract_package() {
 }
 
 termux_step_pre_configure() {
+	CFLAGS+=" -D_POSIX_C_SOURCE=200809L"
 	./autogen.sh
 
 	echo "ac_cv_func_getmntent=no" >> termux_configure.cache