Broken make dependency somewhere near win32ver.o?

Hi,

Here's a strange one-off failure seen on CI[1], in the
CompilerWarnings task where we check that mingw cross-compile works:

[10:48:29.045] time make -s -j${BUILD_JOBS} world-bin
[10:48:38.705] x86_64-w64-mingw32-gcc: error: win32ver.o: No such file
or directory
[10:48:38.705] make[3]: *** [Makefile:44: pg_dumpall] Error 1
[10:48:38.705] make[3]: *** Waiting for unfinished jobs....
[10:48:38.709] make[2]: *** [Makefile:43: all-pg_dump-recurse] Error 2
[10:48:38.709] make[2]: *** Waiting for unfinished jobs....
[10:48:38.918] make[1]: *** [Makefile:42: all-bin-recurse] Error 2
[10:48:38.918] make: *** [GNUmakefile:21: world-bin-src-recurse] Error 2

I guess this implies a dependency problem somewhere around
src/makefiles/Makefile.win32 but I'm not familiar with how that .rc
stuff is supposed to work and I figured I'd mention it here in case
it's obvious to someone else...

[1] https://cirrus-ci.com/task/5546921619095552