Possible to override the definitions created by nimterop? [low priority feature request]
Hello,

This feature request is a spin-off off https://github.com/genotrance/nimterop/issues/50.

I was wondering if something like this can be made to work:

```nim
import nimterop/cimport

# ...

const
  xlmIncludePath = getEnv("XCELIUM_ROOT") / ".." / "include"
static:
  doAssert fileExists(xlmIncludePath / "svdpi.h")
  doAssert fileExists(xlmIncludePath / "svdpi_compatibility.h")

const
  headersvdpiLocal = xlmIncludePath / "svdpi.h"
type
  svOpenArrayHandle* = pointer
proc svGetArrElemPtr*(a1: svOpenArrayHandle; indx1: cint): pointer {.
    importc: "svGetArrElemPtr", header: headersvdpiLocal.}
proc svGetArrElemPtr1*(a1: svOpenArrayHandle; indx1: cint): pointer {.
    importc: "svGetArrElemPtr1", header: headersvdpiLocal.}
proc svGetArrElemPtr2*(a1: svOpenArrayHandle; indx1: cint; indx2: cint): pointer {.
    importc: "svGetArrElemPtr2", header: headersvdpiLocal.}
proc svGetArrElemPtr3*(a1: svOpenArrayHandle; indx1: cint; indx2: cint; indx3: cint): pointer {.
    importc: "svGetArrElemPtr3", header: headersvdpiLocal.}

cAddSearchDir(xlmIncludePath)
cImport(cSearchPath("svdpi.h"))
```

The idea is that `cImport` skips defining `svOpenArrayHandle`, `svGetArrElemPtr`, etc. which already got defined before `cImport` got called.

Without that, right now I get the below expected error:

> /home/kmodi/stow/pkgs/nim/devel/lib/core/macros.nim(528, 3) Error: redefinition of 'svOpenArrayHandle'; previous declaration here: ../../svdpi.nim(22, 3)

So `nimterop` needs to check if an identifier is already defined, and proceed adding its definition only if that identifier doesn't already exist. Is that possible?