Cygwin: cygserver: build with -Wimplicit-fallthrough=5

Define the pseudo keyword 'fallthrough' in woutsup.h to support this.

winsup/cygserver/Makefile.in

@@ -16,7 +16,7 @@ export CXX:=@CXX@
 
 CFLAGS:=@CFLAGS@
 override CXXFLAGS=@CXXFLAGS@
-override CXXFLAGS+=-MMD -Wimplicit-fallthrough=4 -Werror -D__OUTSIDE_CYGWIN__ -DSYSCONFDIR="\"$(sysconfdir)\""
+override CXXFLAGS+=-MMD -Wimplicit-fallthrough=5 -Werror -D__OUTSIDE_CYGWIN__ -DSYSCONFDIR="\"$(sysconfdir)\""
 
 include ${srcdir}/../Makefile.common
 

winsup/cygserver/bsd_helper.cc

@@ -120,7 +120,7 @@ ipcexit_hookthread (const LPVOID param)
     {
       case WAIT_OBJECT_0:
         /* Cygserver shutdown. */
-	/*FALLTHRU*/
+	fallthrough;
       case WAIT_OBJECT_0 + 1:
         /* Process exited.  Call semexit_myhook to handle SEM_UNDOs for the
 	   exiting process and shmexit_myhook to keep track of shared

winsup/cygserver/bsd_mutex.cc

@@ -326,7 +326,7 @@ _msleep (void *ident, struct mtx *mtx, int priority,
         break;
       case WAIT_OBJECT_0 + 1:	/* Shutdown event (triggered by wakeup_all). */
         priority |= PDROP;
-	/*FALLTHRU*/
+	fallthrough;
       case WAIT_OBJECT_0 + 2:	/* The dependent process has exited. */
 	debug ("msleep process exit or shutdown for %d", td->td_proc->winpid);
 	ret = EIDRM;

winsup/cygserver/woutsup.h

@@ -12,6 +12,8 @@ details. */
 #error "woutsup.h is not for code being compiled inside the dll"
 #endif
 
+#define fallthrough	__attribute__((__fallthrough__))
+
 #ifndef _WIN32_WINNT
 #define _WIN32_WINNT 0x0500
 #endif