build: add check for zeromq CURVE support

Problem: when zeromq is configured with --disable-curve, the
flux broker will fail to initialize.

Add a configure time check that zmq_curve_keypair() can be
run successfully.  It fails with ENOTSUPP if CURVE support
is lacking.

Fixes #6151

Author: garlick

config/x_ac_zeromq.m4

@@ -4,5 +4,32 @@ AC_DEFUN([X_AC_ZEROMQ], [
 
     PKG_CHECK_MODULES([ZMQ], [libzmq >= 4.0.4])
 
+    old_CFLAGS=$CFLAGS
+    CFLAGS="$CFLAGS $ZMQ_CFLAGS"
+
+    old_LIBS=$LIBS
+    LIBS="$LIBS $ZMQ_LIBS"
+
+    AC_MSG_CHECKING([whether zeromq has CURVE support])
+    AC_RUN_IFELSE([
+        AC_LANG_SOURCE([[
+            #include <zmq.h>
+	    int main () {
+	        char pub[41];
+	        char sec[41];
+	        if (zmq_curve_keypair (pub, sec) < 0) return 1;
+	        return 0;
+	    }
+	]])],
+	[have_zmq_curve_support=yes; AC_MSG_RESULT([yes])],
+	[AC_MSG_RESULT([no])]
+    )
+
+    CFLAGS=$old_CFLAGS
+    LIBS=$old_LIBS
+
+    AS_IF([test "x$have_zmq_curve_support" != "xyes"],
+        [AC_MSG_ERROR([zeromq CURVE/libsodium support is required])]
+    )
   ]
 )