Merge bitcoin#752: autoconf: Use ":" instead of "dnl" as a noop

5e8747a autoconf: Use ":" instead of "dnl" as a noop (Tim Ruffing)

Pull request description:

  Fixes bitcoin#424.

Top commit has no ACKs.

Tree-SHA512: a83664afbc6ca1254c4767161bfbec82f3489a8a248ba7a5a46ed9ec2a39232cf92f504accadd4dbb1a6ea4791dbf7f0e1f030e51f02f49eb9a38a2e509ee6c2

configure.ac

@@ -210,7 +210,7 @@ if test x"$use_ecmult_static_precomputation" != x"no"; then
   AC_RUN_IFELSE(
     [AC_LANG_PROGRAM([], [])],
     [working_native_cc=yes],
-    [working_native_cc=no],[dnl])
+    [working_native_cc=no],[:])
 
   CFLAGS_FOR_BUILD="$CFLAGS"