Enable SIMD by default (if g++ is around).

NEWS.md

@@ -148,16 +148,19 @@ Protocol: 31 (unchanged)
    target -- rsync-ssl should be considered to be mainstream now that Samba
    requires SSL for its rsync daemon).
 
- - Add _build_ dependency for liblz4-devel, libxxhash-devel, & libzstd-devel.
-   These development libraries will give rsync extra compression and checksum
-   algorithms.
+ - Add _build_ dependency for liblz4-dev, libxxhash-dev, libzstd-dev, and
+   libssl-dev.  These development libraries will give rsync extra compression
+   algorithms, extra checksum algorithms, and allow use of openssl's crypto
+   lib for MD4/MD5 checksums.
 
- - Remove yodl _build_ dependency (if you listed it).
+ - Add _build_ dependency for g++ to enable the SIMD checksum optimizations.
 
  - Add _build_ dependency for _either_ python3-cmarkcfm or python3-commonmark
    to allow for patching of man pages or building a git release.  Note that
    cmarkcfm is faster than commonmark, but they generate the same data.
 
+ - Remove yodl _build_ dependency (if you listed it).
+
 ### DEVELOPER RELATED:
 
  - Silenced some annoying warnings about major()|minor() due to the autoconf

configure.ac

@@ -163,9 +163,9 @@ SIMD=
 
 AC_MSG_CHECKING([whether to enable SIMD optimizations])
 AC_ARG_ENABLE(simd,
-    AS_HELP_STRING([--enable-simd],[enable SIMD optimizations]))
+    AS_HELP_STRING([--disable-simd],[disable SIMD optimizations (requires g++)]))
 
-if test x"$enable_simd" = x"yes"; then
+if test x"$enable_simd" != x"no"; then
     # For x86-64 SIMD, g++ is also required
     if test x"$build_cpu" = x"x86_64" && test x"$CXX" = x"g++"; then
         SIMD="$SIMD x86_64"
@@ -236,7 +236,7 @@ ipv6lib=none
 ipv6trylibc=yes
 
 AC_ARG_ENABLE(ipv6,
-	AS_HELP_STRING([--disable-ipv6],[do not even try to use IPv6]))
+	AS_HELP_STRING([--disable-ipv6],[turn off IPv6 support]))
 if test x"$enable_ipv6" != x"no"; then
 	AC_MSG_CHECKING([ipv6 stack type])
 	for i in inria kame linux-glibc linux-inet6 solaris toshiba v6d zeta cygwin; do