Trac #29489: Fix configure messages "rm: conftest.dir: is a directory"

Some autoconf macro is trying to use `rm -f` when it should be using `rm
-rf`, leading to these harmless error messages:
{{{
checking for ZMQ version >= 4.2.5... yes
rm: conftest.dir: is a directory
rm: conftest_venv: is a directory
configure: will use system package and not install SPKG zeromq
}}}

URL: https://trac.sagemath.org/29489
Reported by: mkoeppe
Ticket author(s): Michael Orlitzky
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/python3/spkg-configure.m4

@@ -149,4 +149,10 @@ EOF
     dnl POST
     AS_IF([test x$sage_spkg_install_python3 = xno], [PYTHON_FOR_VENV="$ac_cv_path_PYTHON3"])
     AC_SUBST([PYTHON_FOR_VENV])
+
+    dnl These temporary directories are created by the check above
+    dnl and need to be cleaned up to prevent the "rm -f conftest*"
+    dnl (that a bunch of other checks do) from emitting warnings about
+    dnl conftest.dir and conftest_venv being directories.
+    rm -rf conftest.dir conftest_venv
 ])