COMP: use standard compiler macros when distinguishing the OS

- replace (darwin) with (__APPLE__)
- replace (solarisGcc) with (__sun__ && __GNUC__)
- instead of 'darwin' -> '__APPLE'

- cease with passing a -D$(WM_ARCH) define since this adds no useful
  additional information and isn't used anywhere.

Reference
http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system

--

COMP: Extend size disambiguation on long (#1238)

src/OpenFOAM/db/dynamicLibrary/dynamicCode/dynamicCode.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           |
+    \\  /    A nd           | Copyright (C) 2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2011 OpenFOAM Foundation
@@ -223,7 +223,7 @@ public:
         //  Corresponds to codeRoot()/libSubDir()/lib\<codeName\>.so
         fileName libPath() const
         {
-            #ifdef darwin
+            #ifdef __APPLE__
             return codeRoot_/libSubDir_/"lib" + codeName_ + ".dylib";
             #else
             return codeRoot_/libSubDir_/"lib" + codeName_ + ".so";