Allow older versions of curl to be used

...at your own risk.  cURL has had some serious security issues over
the years, so if you're using an ancient version, I hope that you're
receiving security patches from your vendor.

The version set here is to accommodate CentOS 6.

configure.ac

@@ -1074,7 +1074,7 @@ case $host in
        ;;
 esac
 
-PKG_CHECK_MODULES([CURL], [libcurl >= 7.26])
+PKG_CHECK_MODULES([CURL], [libcurl >= 7.19])
 PKG_CHECK_MODULES([PCRE], [libpcre >= 7.8])
 PKG_CHECK_MODULES([SPEEX], [speex >= 1.2rc1 speexdsp >= 1.2rc1])