Fix pod2man build error

When trying to build ldm in a clean chroot/system, the man page is broken:
```
cc -O2 -march=x86-64 -mtune=generic -O2 -pipe -fstack-protector --param=ssp-buffer-size=4 -o ldm.o -c ldm.c
make: git: Command not found
cc -ludev -lmount  -o ldm ldm.o
make: git: Command not found
install -D -m 644 ldm.service /build/ldm/pkg/ldm/usr/lib/systemd/system/ldm.service
install -D -m 755 ldm /build/ldm/pkg/ldm/usr/bin/ldm
install -D -m 644 ldm.1 /build/ldm/pkg/ldm/usr/share/man/man1/ldm.1
```

Add `VERSION` to Makefile.
The version number is now determined using the `VERSION_STR` variable. No more git needed.
Just increasing the variable above automatically raise the version number in the Makefile too.