compiler/cgen, ccgthreadvars: export variables correctly

For C/C++ variables, `extern` means that the variable is defined in an
another unit. Added a new N_LIB_EXPORT_VAR to correctly export
variables.

compiler/ccgthreadvars.nim

@@ -33,7 +33,7 @@ proc declareThreadVar(m: BModule, s: PSym, isExtern: bool) =
       m.g.nimtv.addf("$1 $2;$n", [getTypeDesc(m, s.loc.t), s.loc.r])
   else:
     if isExtern: m.s[cfsVars].add("extern ")
-    elif lfExportLib in s.loc.flags: m.s[cfsVars].add("N_LIB_EXPORT ")
+    elif lfExportLib in s.loc.flags: m.s[cfsVars].add("N_LIB_EXPORT_VAR ")
     else: m.s[cfsVars].add("N_LIB_PRIVATE ")
     if optThreads in m.config.globalOptions: m.s[cfsVars].add("NIM_THREADVAR ")
     m.s[cfsVars].add(getTypeDesc(m, s.loc.t))

compiler/cgen.nim

@@ -549,7 +549,7 @@ proc assignGlobalVar(p: BProc, n: PNode; value: Rope) =
           decl.addf "NIM_ALIGN($1) ", [rope(s.alignment)]
         if p.hcrOn: decl.add("static ")
         elif sfImportc in s.flags: decl.add("extern ")
-        elif lfExportLib in s.loc.flags: decl.add("N_LIB_EXPORT ")
+        elif lfExportLib in s.loc.flags: decl.add("N_LIB_EXPORT_VAR ")
         else: decl.add("N_LIB_PRIVATE ")
         if s.kind == skLet and value != nil: decl.add("NIM_CONST ")
         decl.add(td)

lib/nimbase.h

@@ -186,6 +186,7 @@ __AVR__
 #  else
 #    define N_LIB_EXPORT  NIM_EXTERNC __declspec(dllexport)
 #  endif
+#  define N_LIB_EXPORT_VAR  __declspec(dllexport)
 #  define N_LIB_IMPORT  extern __declspec(dllimport)
 #else
 #  define N_LIB_PRIVATE __attribute__((visibility("hidden")))
@@ -215,6 +216,7 @@ __AVR__
 #    define N_SAFECALL_PTR(rettype, name) rettype (*name)
 #  endif
 #  define N_LIB_EXPORT NIM_EXTERNC __attribute__((visibility("default")))
+#  define N_LIB_EXPORT_VAR  __attribute__((visibility("default")))
 #  define N_LIB_IMPORT  extern
 #endif