configure: configure the native build of protoc with HOST name in contribs

Similar to how we force detection of luac there.

We might be using
AC_CHECK_TOOL(PROTOC, protoc, no, [${CONTRIB_DIR}/../bin$PATH_SEPARATOR$PATH])

Which works, but it doesn't keep the full path, so if the PATH is not set in
the environment when using HOST-protoc it doesn't find it. So we either have to
force everyone to add a new item in their PATH or just use the transparent way
that luac is using.

configure.ac

@@ -569,6 +569,16 @@ AS_IF([test -n "${CONTRIB_DIR}"], [
     ])
   ])
 
+  AS_IF([test -z "$PROTOC"], [
+    AC_MSG_CHECKING([if contribs provide protoc])
+    AS_IF([test -x "${CONTRIB_DIR}/../bin/${host_alias}-protoc${BUILDEXEEXT}"], [
+      PROTOC="${CONTRIB_DIR}/../bin/${host_alias}-protoc${BUILDEXEEXT}"
+      AC_MSG_RESULT([yes])
+    ],[
+      AC_MSG_RESULT([no])
+    ])
+  ])
+
   AS_IF([test "${SYS}" = "darwin"], [
     export LD_LIBRARY_PATH="${CONTRIB_DIR}/lib:$LD_LIBRARY_PATH"
     export DYLD_LIBRARY_PATH="${CONTRIB_DIR}/lib:$DYLD_LIBRARY_PATH"