include: add _NL_NO_WARN_DEPRECATED_HEADER for suppressing warning ab…

…out deprecated headers

Header files are part of a stable API. Warning about using deprecated
API is cumbersome, albeit often useful to force the user to migrate
away from the API.

But it also requires that the unit test disables "-Wcpp". That is
problematic on its own, because we don't want to disable any warnings.

As we only have 4 such header files, instead guard the warnings with
a

  #ifndef _NL_NO_WARN_DEPRECATED_HEADER

Makefile.am

@@ -1142,9 +1142,9 @@ EXTRA_DIST += \
 	$(COMPILE) \
 		-Wall \
 		-Werror \
-		-Wno-error=cpp \
 		-I$(srcdir)/include \
 		-I$(builddir)/include \
+		-D_NL_NO_WARN_DEPRECATED_HEADER \
 		-c \
 		-o $@ $<
 

include/netlink/cache-api.h

@@ -9,6 +9,8 @@
 #include <netlink/netlink.h>
 #include <netlink/cache.h>
 
+#ifndef _NL_NO_WARN_DEPRECATED_HEADER
 #warning "You are including a deprecated header file, include <netlink/cache.h>."
+#endif
 
 #endif

include/netlink/route/link/api.h

@@ -9,6 +9,8 @@
 #include <netlink/netlink.h>
 #include <netlink/route/link.h>
 
+#ifndef _NL_NO_WARN_DEPRECATED_HEADER
 #warning "You are including a deprecated header file, include <netlink/route/link.h>."
+#endif
 
 #endif

include/netlink/route/link/info-api.h

@@ -9,6 +9,8 @@
 #include <netlink/netlink.h>
 #include <netlink/route/link.h>
 
+#ifndef _NL_NO_WARN_DEPRECATED_HEADER
 #warning "You are including a deprecated header file, include <netlink/route/link.h>."
+#endif
 
 #endif

include/netlink/route/tc-api.h

@@ -10,6 +10,8 @@
 #include <netlink/msg.h>
 #include <netlink/route/tc.h>
 
+#ifndef _NL_NO_WARN_DEPRECATED_HEADER
 #warning "You are including a deprecated header file, include <netlink/route/tc.h>."
+#endif
 
 #endif