adapt to promise api
If there is no smina binary, then it did not successfully compile. Perhaps look more carefully for an error message?
It should be in the build directory with the makefile, e.g. smina/build/linux/release
Looking at the source on github (https://github.com/openbabel/openbabel/blob/master/include/openbabel/plugin.h) those are all lines that should be commented out (there should be more than one slash).
Seems like a cygwin specific problem. I found this by googling "strcasecomp cygwin": https://stackoverflow.com/questions/7248509/strcasecmp-was-not-declared-in-this-scope?rq=1
You need to specify the include path to the openbabel install with -I. Not sure that will be enough to compile under cygwin though - never tried it.
improve some messages
more robust pml parsing