build-package.sh: set ac_cv_func_posix_spawn=no for all autotools builds

posix_spawn is not available on Android devices.

While this commit is not necessary for cross-compiling, it is required
when building on-device since there posix_spawn is found somehow. This
produces following errors:

 ./spawn.h:514:17: error: use of undeclared identifier 'POSIX_SPAWN_RESETIDS'
             [(((POSIX_SPAWN_RESETIDS | POSIX_SPAWN_SETPGROUP

scripts/build/configure/termux_step_configure_autotools.sh

@@ -51,6 +51,7 @@ termux_step_configure_autotools() {
 	AVOID_GNULIB+=" ac_cv_func_chown_works=yes"
 	AVOID_GNULIB+=" ac_cv_func_getgroups_works=yes"
 	AVOID_GNULIB+=" ac_cv_func_malloc_0_nonnull=yes"
+	AVOID_GNULIB+=" ac_cv_func_posix_spawn=no"
 	AVOID_GNULIB+=" ac_cv_func_realloc_0_nonnull=yes"
 	AVOID_GNULIB+=" am_cv_func_working_getline=yes"
 	AVOID_GNULIB+=" gl_cv_func_dup2_works=yes"