maint: allow bootstrap in a sandbox

Jiri Denemark reported an instance of bootstrapping libvirt
failing when run inside a sandbox, traced to rpm trying to
access /var/ which was not permitted by the sandbox.

Alex Jia reported that 0.9.8-rc1 failed to bootstrap if patch(1)
is not installed.

* bootstrap.conf (buildreq): Avoid rpm call if python-config
exists.  Also, require patch, in case we have gnulib-local diffs.

bootstrap.conf

@@ -195,6 +195,7 @@ gettext    0.17
 git        1.5.5
 gzip       -
 libtool    -
+patch      -
 perl       5.5
 pkg-config -
 python-config -
@@ -203,10 +204,11 @@ tar        -
 xmllint	   -
 xsltproc   -
 "
-# You don't have to be on a system with rpm; rather, if you happen to
-# be on RHEL 5, then this bypasses the bootstrap logic that probes for
-# a working 'python-config --version'.
-if `(rpm -q python-devel) >/dev/null 2>&1`; then
+# Use rpm as a fallback to bypass the bootstrap probe for python-config,
+# for the sake of RHEL 5; without requiring it on newer systems that
+# have python-config to begin with.
+if `(${PYTHON_CONFIG-python-config} --version;
+     test $? -lt 126 || rpm -q python-devel) >/dev/null 2>&1`; then
   PYTHON_CONFIG=true
 fi