Fix #4, fix #7: require regex-posix-clib under windows directly

The current solution, to lead the absence of flag _regex-posix-clib
under os(windows) to absurdity (base<0) is not handled correctly by stack.

Author: andreasabel

regex-posix.cabal

@@ -51,7 +51,7 @@ source-repository this
 flag _regex-posix-clib
   manual: False
   default: False
-  description: Use <//hackage.haskell.org/package/regex-posix-clib regex-posix-clib> package
+  description: Use <//hackage.haskell.org/package/regex-posix-clib regex-posix-clib> package (used by default on Windows)
 
 library
   hs-source-dirs: src
@@ -69,13 +69,10 @@ library
   c-sources:        cbits/myfree.c
   include-dirs:     cbits
 
-  if flag(_regex-posix-clib)
+  if flag(_regex-posix-clib) || os(windows)
     build-depends: regex-posix-clib == 2.7.*
-  else
-    -- use POSIX.2 regex implementation from @libc@
-    -- However, Windows/msys2 doesn't provide a POSIX.2 regex impl in its @libc@
-    if os(windows)
-      build-depends: base<0
+    -- Otherwise, use POSIX.2 regex implementation from @libc@.
+    -- However, Windows/msys2 doesn't provide a POSIX.2 regex impl in its @libc@.
 
   default-language: Haskell2010
   default-extensions: