Implement relative import support in header files.

You must currently replace relative imports using nimgen's commands,
but relative files don't crash nimgen anymore.

nimgen.nim

@@ -156,6 +156,9 @@ proc getKey(ukey: string): tuple[key: string, val: bool] =
 # File loction
 
 proc getNimout(file: string, rename=true): string =
+  if file == "":
+    return ""
+
   result = file.splitFile().name.replace(re"[\-\.]", "_") & ".nim"
   if gOutput != "":
     result = gOutput/result
@@ -193,6 +196,8 @@ proc search(file: string): string =
       echo "File doesn't exist: " & file
       quit(1)
 
+  # Only keep relative directory
+  result = result.replace(gProjectDir & $DirSep, "")
   return result.replace(re"[\\/]", $DirSep)
 
 # ###
@@ -342,12 +347,20 @@ proc getIncls(file: string, inline=false): seq[string] =
   if inline and file in gDoneInline:
     return
 
+  var curPath = splitFile(expandFileName(file))[0]
   withFile(file):
     for f in content.findIter(re"(?m)^\s*#\s*include\s+(.*?)$"):
       var inc = f.captures[0].strip()
       if ((gQuotes and inc.contains("\"")) or (gFilter != "" and gFilter in inc)) and (not exclude(inc)):
-        result.add(
-          inc.replace(re"""[<>"]""", "").replace(re"\/[\*\/].*$", "").strip())
+        var addInc = inc.replace(re"""[<>"]""", "").replace(re"\/[\*\/].*$", "").strip()
+        try:
+          # Try searching for a local library
+          let finc = expandFileName(curPath & "/" & addInc)
+          let fname = extractFileName(finc)
+          result.add(fname.search())
+        except OSError:
+          # If it's a system library
+          result.add(addInc)
 
     result = result.deduplicate()
 
@@ -469,7 +482,9 @@ proc c2nim(fl, outfile: string, c2nimConfig: c2nimConfigObj) =
 
     for inc in incls:
       runFile(inc, cfg)
-      incout &= "import $#\n" % inc.search().getNimout()[0 .. ^5]
+      let nimout = inc.search().getNimout()
+      if nimout.len() > 0:
+        incout &= "import $#\n" % nimout[0 .. ^5]
 
   var cfile = file
   if c2nimConfig.preprocess: