Require Python for RPC tests, when using lcov

Because Python is (going to be) used to run the RPC tests, when
gathering coverage data with lcov, it is explicitly checked, whether
Python is really available.

configure.ac

@@ -58,6 +58,7 @@ AC_PATH_TOOL(STRIP, strip)
 AC_PATH_TOOL(GCOV, gcov)
 AC_PATH_PROG(LCOV, lcov)
 AC_PATH_PROG(JAVA, java)
+AC_PATH_PROG(PYTHON, python)
 AC_PATH_PROG(GENHTML, genhtml)
 AC_PATH_PROG([GIT], [git])
 AC_PATH_PROG(CCACHE,ccache)
@@ -351,6 +352,9 @@ if test x$use_lcov = xyes; then
   if test x$JAVA = x; then
     AC_MSG_ERROR("lcov testing requested but java not found")
   fi
+  if test x$PYTHON = x; then
+    AC_MSG_ERROR("lcov testing requested but python not found")
+  fi
   if test x$GENHTML = x; then
     AC_MSG_ERROR("lcov testing requested but genhtml not found")
   fi