cIncludeDir doesn't seem to be working
Hello,

I am using nimterop built from master and nim built from devel.

I am using nimterop this way:

```nim
import nimterop/cimport

cDebug()

# Below doesn't work
cIncludeDir("/cad/adi/apps/cadence/xlm/linux/current/tools/include")
# Below doesn't work
# cIncludeDir(/cad/adi/apps/cadence/xlm/linux/current/tools/include)
# Below doesn't work
# {.passC: "-I/cad/adi/apps/cadence/xlm/linux/current/tools/include".}

# Below manual definition of s_vpi_vecval is a workaround for
# https://github.com/genotrance/nimterop/issues/47.
type
  s_vpi_vecval* {.importc: "s_vpi_vecval".} = object
    aval: uint32
    bval: uint32

cImport("svdpi.h")
```

The `svdpi.h` and the other header file needed are in that directory path that I have in the above example, but still they cannot be found.  I get this error:

```
{.passC: "-I\"/cad/adi/apps/cadence/xlm/linux/current/tools/include\"".}
stack trace: (most recent call last)
../../../../.nimble/pkgs/nimterop-0.1.0/nimterop/cimport.nim(232, 24) cImport
../../../../.nimble/pkgs/nimterop-0.1.0/nimterop/cimport.nim(21, 11) findPath
../../../../stow/pkgs/nim/devel/lib/system.nim(3989, 20) failedAssertImpl
../../../../stow/pkgs/nim/devel/lib/system.nim(3982, 11) raiseAssert
../../../../stow/pkgs/nim/devel/lib/system.nim(3016, 7) sysFatal
svdpi.nim(16, 8) template/generic instantiation of `cImport` from here
../../../../stow/pkgs/nim/devel/lib/system.nim(3016, 7) Error: unhandled exception: /home/kmodi/.nimble/pkgs/nimterop-0.1.0/nimterop/cimport.nim(21, 5) `(not fail)` File or directory not found: svdpi.h
```

The only way I can get the compilation to work is if I copy the .h files and put them in the directory containing the above .nim file.

Am I missing something in the `cIncludeDir` syntax? 