lavd/decklink: Fix compile issue on OS X

Fixes #4124: Invalid argument '-std=c99' not allowed with 'C++/ObjC++'
C++ files fail to compile. This adds '-std=c++11' to CXX_FLAGS to fix.
Signed-off-by: Rick Kern <kernrj@gmail.com>

common.mak

@@ -39,7 +39,7 @@ CCFLAGS     = $(CPPFLAGS) $(CFLAGS)
 OBJCFLAGS  += $(EOBJCFLAGS)
 OBJCCFLAGS  = $(CPPFLAGS) $(CFLAGS) $(OBJCFLAGS)
 ASFLAGS    := $(CPPFLAGS) $(ASFLAGS)
-CXXFLAGS   += $(CPPFLAGS) $(CFLAGS)
+CXXFLAGS   := $(CPPFLAGS) $(CFLAGS) $(CXXFLAGS)
 YASMFLAGS  += $(IFLAGS:%=%/) -Pconfig.asm
 
 HOSTCCFLAGS = $(IFLAGS) $(HOSTCPPFLAGS) $(HOSTCFLAGS)

configure

@@ -4519,6 +4519,7 @@ fi
 
 add_cppflags -D_ISOC99_SOURCE
 add_cxxflags -D__STDC_CONSTANT_MACROS
+add_cxxflags -std=c++11
 check_cflags -std=c99
 check_cc -D_FILE_OFFSET_BITS=64 <<EOF && add_cppflags -D_FILE_OFFSET_BITS=64
 #include <stdlib.h>