[PATCH 1/3] build: Check for -Wimplicit-fallthrough=5 separately

[tbaeder@redhat.com]

From: Timm Bäder <tbaeder@redhat.com>

GCC accepts the =5, which means it doesn't try to parse any comments
and only accepts the fallthrough attribute in code. Clang does not ever
parse any comments and always wants the fallthrough attribute anyway.
Clang also doesn't accept the =n parameter for -Wimplicit-fallthrough.

Test for =5 separately and use it if supported and fall back to just
-Wimplicit-fallthrough otherwise.

Signed-off-by: Timm Bäder <tbaeder@redhat.com>
---
 config/eu.am |  4 ++++
 configure.ac | 12 ++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/config/eu.am b/config/eu.am
index 6c3c444f..e109ffd3 100644
--- a/config/eu.am
+++ b/config/eu.am
@@ -64,8 +64,12 @@ endif
 if HAVE_IMPLICIT_FALLTHROUGH_WARNING
 # Use strict fallthrough. Only __attribute__((fallthrough)) will prevent the
 # warning
+if HAVE_IMPLICIT_FALLTHROUGH_5_WARNING
 IMPLICIT_FALLTHROUGH_WARNING=-Wimplicit-fallthrough=5
 else
+IMPLICIT_FALLTHROUGH_WARNING=-Wimplicit-fallthrough
+endif
+else
 IMPLICIT_FALLTHROUGH_WARNING=
 endif
 
diff --git a/configure.ac b/configure.ac
index d345495d..e56aeb6a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -519,6 +519,18 @@ CFLAGS="$old_CFLAGS"])
 AM_CONDITIONAL(HAVE_IMPLICIT_FALLTHROUGH_WARNING,
 	       [test "x$ac_cv_implicit_fallthrough" != "xno"])
 
+# Check whether the compiler additionally accepts -Wimplicit-fallthrough=5
+# GCC accepts this and 5 means "don't parse any fallthrough comments and
+# only accept the fallthrough attribute"
+AC_CACHE_CHECK([whether the compiler accepts -Wimplicit-fallthrough=5], ac_cv_implicit_fallthrough_5, [dnl
+old_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Wimplicit-fallthrough=5 -Werror"
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([])],
+		  ac_cv_implicit_fallthrough_5=yes, ac_cv_implicit_fallthrough_5=no)
+CFLAGS="$old_CFLAGS"])
+AM_CONDITIONAL(HAVE_IMPLICIT_FALLTHROUGH_5_WARNING,
+	       [test "x$ac_cv_implicit_fallthrough_5" != "xno"])
+
 # Assume the fallthrough attribute is supported if -Wimplict-fallthrough is supported
 if test "$ac_cv_implicit_fallthrough" = "yes"; then
 	AC_DEFINE([HAVE_FALLTHROUGH], [1],
-- 
2.26.2