conf: Generate and install config sippets for option descriptions

The strongswan.d directory is also created relative to the configured
location of strongswan.conf.

configure.ac

@@ -302,6 +302,7 @@ AC_PROG_EGREP
 AC_PROG_AWK
 AC_PROG_LEX
 AC_PROG_YACC
+AM_PATH_PYTHON(,,[:])
 AC_PATH_PROG([PERL], [perl], [], [$PATH:/bin:/usr/bin:/usr/local/bin])
 AC_PATH_PROG([GPERF], [gperf], [], [$PATH:/bin:/usr/bin:/usr/local/bin])