autoconf: Use ":" instead of "dnl" as a noop

Summary:
Fixes #424.

This is a backport of libsecp256k1 [[bitcoin-core/secp256k1#752 | PR752]]

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7588

src/secp256k1/configure.ac

@@ -225,7 +225,7 @@ 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"