Problem: cannot disable Werror with autoconf

Solution: add --disable-Werror flag like the existing
--disable-pedantic or CMake's -DLIBZMQ_WERROR=OFF
Fixes #2818

NEWS

@@ -79,6 +79,9 @@
 * Add new autoconf --disable-libunwind option to stop building with libunwind
   even if it is available.
 
+* Add new autoconf --disable-Werror option to avoid building with the Werror
+  flag.
+
 * Use pkg-config as the first method for finding and building with external
   optional dependencies such as libnorm, libpgm and gssapi.
 

configure.ac

@@ -144,8 +144,11 @@ else
     AC_MSG_RESULT([no])
 fi
 
-# By default compiling with -Werror except OSX.
-libzmq_werror="yes"
+# By default compiling with -Werror except OSX and on Solaris when building
+# with libsodium.
+AC_ARG_ENABLE([Werror],
+    [AS_HELP_STRING([--disable-Werror], [disable Werror compiler flag [default=enabled]])],
+    [libzmq_werror=$enableval], [libzmq_werror=yes])
 
 # By default use DSO visibility
 libzmq_dso_visibility="yes"