CMake,autotools: enable -Werror=implicit by default

The -Wimplicit error covers two warnings, trying to catch these cases:

Setting a global variable without declaring its type (-Wimplicit-int):

    undeclared_type = 1;

More importantly, -Wimplicit-function-declaration catches the case where
a function is not declared (missing header, programming error, etc.).

Turn these warnings into errors, most likely it will be a programming
error that results in a link failure anyway. See also
https://fedoraproject.org/wiki/Changes/Fedora26CFlags

Also fix autotools checks not to trigger -Wimplicit-int and
-Wimplicit-function-declaration (in krb5 check due to missing include).
Tested on Ubuntu 16.04 (autotools & cmake) and Arch Linux (cmake),
configure/cmake output and config.h are identical.

Change-Id: I137284263f3b1223df6e6a893111c3640802631f
Reviewed-on: https://code.wireshark.org/review/19331
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

1 files changed, 7 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 13ed6076f0..697b6c0c04 100644
--- a/configure.ac
+++ b/configure.ac
@@ -657,6 +657,13 @@ AC_WIRESHARK_COMPILER_FLAGS_CHECK(-Wjump-misses-init, C)
 # so only enable this for C for now.
 AC_WIRESHARK_COMPILER_FLAGS_CHECK(-Wshorten-64-to-32, C)
 
+#
+# Implicit function declarations are an error in C++ and most
+# likely a programming error in C. Turn -Wimplicit-int and
+# -Wimplicit-function-declaration into an error by default.
+#
+AC_WIRESHARK_COMPILER_FLAGS_CHECK(-Werror=implicit, C)
+
 # Clang only. Avoid "argument unused during compilation" warnings
 # (for example, when getting the -gsplit-dwarf option or
 # when combining -fwrapv with -fno-strict-overflow)