`getHeader` doesn't find a header
While looking for a workaround for issue https://github.com/nimterop/nimterop/issues/257, it looks like `getheader` might not be managing well header files with symbols like `_`:

I have the file **occt_wrapper.nim**:
```nim
# nim c -d:Geom2dCartesianPointStd occt_wrapper
import nimterop/[build, cimport]

static:
  cDebug()                                                # Print wrapper to stdout

cIncludeDir("/usr/include/opencascade")

# The following should create the variables: "Geom2d_CartessianPointPath" 
# and "Geom2d_CartessianPointLPath"
getHeader( "opencascade/Geom2d_CartesianPoint.hxx", altNames = "TKG2d", outdir= "/usr/lib" )

# Checking if nim was called with "-d:headerStatic"
when not isDefined(headerStatic):
  c2nImport(Geom2d_CartesianPointPath, recurse = true, mode="cpp", dynlib = Geom2d_CartesianPointLPath)       # Pass dynlib if not static link
else:
  c2nImport(Geom2d_CartesianPointPath, recurse = true, mode="cpp")
```
but when I compile:
```
nim c -d:Geom2dCartesianPointStd occt_wrapper
```

I get the error in the `getHeader` line:
```
/home/jose/.choosenim/toolchains/nim-1.2.6/lib/system/fatal.nim(49, 5) Error: unhandled exception: /home/jose/.nimble/pkgs/nimterop-#head/nimterop/build/getheader.nim(506, 16) `Geom2dCartesianPointPath.len != 0` 
Header opencascade/Geom2d_CartesianPoint.hxx not found - missing/empty outdir or -d:Geom2dCartesianPointStd -d:Geom2dCartesianPointGit -d:Geom2dCartesianPointDL -d:Geom2dCartesianPointConan or -d:Geom2dCartesianPointJBB not specified [AssertionError]
```

I have tried using the header name: `/usr/include/opencascade/Geom2d_CartesianPoint.hxx` and I get the error:
```
Header /usr/include/opencascade/Geom2d_CartesianPoint.hxx not found - missing/empty outdir or -d:Geom2dCartesianPointStd -d:Geom2dCartesianPointGit -d:Geom2dCartesianPointDL -d:Geom2dCartesianPointConan or -d:Geom2dCartesianPointJBB not specified [AssertionError]
```
but that file obviously exists.
