Lower required version of sqlite for CentOS 6

Apparently CentOS 6 has only v3.6.20 of sqlite, so we'll accommodate
that barring evidence we shouldn't.  Please test and report back.

configure.ac

@@ -1081,7 +1081,7 @@ case $host in
        ;;
 esac
 
-PKG_CHECK_MODULES([SQLITE], [sqlite3 >= 3.7.13])
+PKG_CHECK_MODULES([SQLITE], [sqlite3 >= 3.6.20])
 PKG_CHECK_MODULES([CURL], [libcurl >= 7.19])
 PKG_CHECK_MODULES([PCRE], [libpcre >= 7.8])
 PKG_CHECK_MODULES([SPEEX], [speex >= 1.2rc1 speexdsp >= 1.2rc1])