Improve relative path check for msi

For posix-ish systems, we previously checked that a path was relative
by simpy checking if it included a '/' character. This meant that you
could not, for example, do
```
$ cat foo.substitutions
file rel/to/bar.template {
}

$ msi -I /some/path foo.substitutions
```
where our template file is located at `/some/path/rel/to/bar.template`.

Note that relateive paths work differently on Windows, so we carve out
an exception there.

modules/database/src/ioc/dbtemplate/Makefile

@@ -14,6 +14,7 @@ SRC_DIRS += $(IOCDIR)/dbtemplate
 PROD_HOST += msi
 
 msi_SRCS = msi.cpp
+msi_SYS_LIBS_WIN32 = shlwapi
 DOCS += msi.md
 
 INC += dbLoadTemplate.h