waf: fix concurrency issue with duplicate targets

waf: fix concurrency issue with duplicate targets

The same source file cannot safely be used in different wscripts
with concurrency (if they were in the same wscript it works
.c.1.o .c.2.o, etc).

[387/764] c: libs/fst/vstwin.c -> build/libs/fst/vstwin.c.1.o
[731/764] c: libs/fst/vstwin.c -> build/libs/fst/vstwin.c.1.o
[746/764] cxxprogram: build/libs/fst/scanner.cc.1.o build/libs/fst/vstwin.c.1.o -> build/libs/fst/ardour-vst-scanner.exe
[750/764] cxxshlib: [...]  build/libs/fst/vstwin.c.1.o [...] -> build/libs/ardour/ardour-3.dll

and about one every full moon (depending on concurrency) it failed:
missing file: 'build/libs/fst/vstwin.c.1.o'