[tlbuild] gregorio doesn't build

[Ken Brown]

Building the current TL sources fails as follows:

make[4]: *** No rule to make target 'gregorio-4_2_0.exe', needed by 
'all-am'.  Stop.

I'm no autotools expert, but I think the problem might be that 
bin_PROGRAMS in texk/gregorio/Makefile.am was changed from gregorio to 
gregorio-$(FILENAME_VERSION) in the recent commit, but Makefile.am still 
defines gregorio_SOURCES instead of something involving FILENAME_VERSION.

Ken