Only use -Wstrict-prototypes with gcc 3.3 or later.

It is working with earlier versions, but when I use it with 3.0.3, I can't
get it to ignore errors in "system headers" with -isystem so we get
excessive amounts of warnings on SSL headers which is very annoying.

configure.ac

@@ -1203,7 +1203,7 @@ AC_HELP_STRING([--disable-debug],[Disable debug options]),
          dnl we only like gcc less than 5.0, since if it is above that it is
          dnl likely just a compiler that looks like gcc (like icc 8.0)!
          dnl here's the standard setup
-         WARN="-W -Wall -Wwrite-strings -pedantic -Wno-long-long -Wno-format-nonliteral -Wundef -Wpointer-arith -Wnested-externs -Winline -Wstrict-prototypes -Wmissing-declarations -Wmissing-prototypes -Wsign-compare"
+         WARN="-W -Wall -Wwrite-strings -pedantic -Wno-long-long -Wno-format-nonliteral -Wundef -Wpointer-arith -Wnested-externs -Winline -Wmissing-declarations -Wmissing-prototypes -Wsign-compare"
 
          dnl -Wcast-align is a bit too annoying ;-)
 
@@ -1215,7 +1215,7 @@ AC_HELP_STRING([--disable-debug],[Disable debug options]),
            dnl on i686-Linux as it gives us heaps with false positives
            if test "$gccnum" -ge "303"; then
              dnl gcc 3.3 and later
-             WARN="$WARN -Wendif-labels"
+             WARN="$WARN -Wendif-labels -Wstrict-prototypes"
            fi
          fi