gstinfo: don't assume G_HAVE_ISO_VARARGS implies ISO C99
gstinfo: don't assume G_HAVE_ISO_VARARGS implies ISO C99

Makes headers C++ clean, esp. with the Sun compilers.
Fixes #567692.

drothlis/gstreamer,Distrotech/gstreamer,drothlis/gstreamer,ylatuya/gstreamer,mparis/gstreamer,Lachann/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,krieger-od/gstreamer,justinjoy/gstreamer,ylatuya/gstreamer,justinjoy/gstreamer,krichter722/gstreamer,mrchapp/gstreamer,ahmedammar/platform_external_gst_gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,collects/gstreamer,jpakkane/gstreamer,collects/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,centricular/gstreamer,mparis/gstreamer,krieger-od/gstreamer,ensonic/gstreamer,centricular/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,lubosz/gstreamer,krichter722/gstreamer,ahmedammar/platform_external_gst_gstreamer,shelsonjava/gstreamer,mrchapp/gstreamer,magcius/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,Distrotech/gstreamer,cablelabs/gstreamer,ahmedammar/platform_external_gst_gstreamer,cfoch/gstreamer,collects/gstreamer,jpxiong/gstreamer,mparis/gstreamer,Lachann/gstreamer,surround-io/gstreamer,mrchapp/gstreamer,krichter722/gstreamer,ahmedammar/platform_external_gst_gstreamer,StreamUtils/gstreamer,ylatuya/gstreamer,magcius/gstreamer,drothlis/gstreamer,magcius/gstreamer,ensonic/gstreamer,ylatuya/gstreamer,collects/gstreamer,ylatuya/gstreamer,cablelabs/gstreamer,drothlis/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,magcius/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,Lachann/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,justinjoy/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,Distrotech/gstreamer,cablelabs/gstreamer,lovebug356/gstreamer,shelsonjava/gstreamer,mrchapp/gstreamer,magcius/gstreamer,jpakkane/gstreamer,ahmedammar/platform_external_gst_gstreamer,drothlis/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,Distrotech/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,Lachann/gstreamer,lubosz/gstreamer,krichter722/gstreamer,surround-io/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,centricular/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,mrchapp/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,Distrotech/gstreamer,lovebug356/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,cfoch/gstreamer,lubosz/gstreamer,cfoch/gstreamer,centricular/gstreamer,lovebug356/gstreamer,collects/gstreamer