Currently, net-mgmt/ettercap does not compile with clang.  This is due to bugs in the configure script.  The configure script contains C code looking like this:

int main() {
	int main(int argc, char *argv[] {
		/* test code here */
		return 0;
	}
	return 0;
}

This code is not accepted by clang (but accepted by gcc, for some reason).

Fix: The files patch-aclocal.m4 and patch-acinclude.m4 contains fixes to the m4 files used to generate the configure script.  They can be used by patching the corresponding files in work/ettercap and run ./autogen.sh. The patch-configure is the resulting changes to the configure script from this, and it also includes the changes already in the current patch file files/patch-configure.
To fix the port, either use the new patch-configure, or use the patched *.m4 files and run autogen.sh during make install.  The latter suggestion has the drawbak of making ettercap require autoconf and automake.

--- acinclude.m4.orig	2012-03-24 18:22:51.000000000 +0100
+++ acinclude.m4	2012-03-24 18:23:49.000000000 +0100
@@ -56,11 +56,8 @@
          #include <pthread.h>
          ],
          [
-            int main(int argc, char **argv)
-            {
                pthread_create(NULL, NULL, NULL, NULL);
                return 0;
-            }
          ],
          [AC_MSG_RESULT(yes)],
          [AC_MSG_RESULT(no)
@@ -182,7 +179,7 @@
    AC_TRY_RUN([
       #include <arpa/nameser.h>
 
-      int main()
+      int main(int argc, char *argv[])
       {
          int i;
          char *p = "\x01\x02\x03\x04";
@@ -214,13 +211,10 @@
                #include <resolv.h>
             ],
             [
-               int main(int argc, char **argv)
-               {
                   char *q;
                   char p[NS_MAXDNAME];
 
                   dn_expand(q, q, q, p, sizeof(p));
-               } 
             ],
             [AC_MSG_RESULT(not needed)],
             [AC_MSG_RESULT(needed)
@@ -239,13 +233,10 @@
                #include <resolv.h>
             ],
             [
-               int main(int argc, char **argv)
-               {
                   char *q;
                   char p[NS_MAXDNAME];
 
                   __dn_expand(q, q, q, p, sizeof(p));
-               } 
 	    ],
 	    [AC_MSG_RESULT(not needed)],
             [AC_MSG_RESULT(needed)
@@ -274,13 +265,10 @@
                #include <resolv.h>
             ],
             [
-               int main(int argc, char **argv)
-               {
                   char *q;
                   char p[NS_MAXDNAME];
 
                   __dn_expand(q, q, q, p, sizeof(p));
-               } 
             ],
             [AC_MSG_RESULT(not needed)],
             [AC_MSG_RESULT(needed)
--- patch-acinclude.m4 ends here ---
How-To-Repeat: 	compile net-mgmt/ettercap with clang instead of gcc.