Trac #29489: clean up conftest.dir and conftest_venv after use.

The python3 spkg-configure.m4 creates two temporary directories
called conftest.dir and conftest_venv, but fails to clean them
up. This would normally be harmless, except that several other
autoconf macros try to "rm -f conftest*", which spits out a warning
when it encounters a directory. Since we can't fix the macros that
we didn't write, this commit modifies the post-check phase of the
python3 SAGE_SPKG_CONFIGURE to clean up its own mess.

Author: orlitzky

build/pkgs/python3/spkg-configure.m4

@@ -93,4 +93,10 @@ 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
 ])