GDK-Broadway build improvements for Visual Studio

Hi,

This updates the build files for GDK-Broadway in the following manner:

• Correct the build variable in gdk/broadway/Makefile.am, where variables should be enclosed with a $(), not a ${}, in a previous commit from me, for consistency with the rest of the sources.
• Support (re-)generating broadwayjs.h and clienthtml.h in 'gdk/broadway' in the NMake Makefile snippets.

With blessings, thank you!