Fix cabal_macros.h for package names containing '-'

As with the Paths_pkgname module, we map '-' to '_' as the
former is not a valid character in cpp macro identifiers.
Fixes cpp redefinition warnings. First reported by gwern.

Distribution/Simple/Build/Macros.hs

@@ -40,7 +40,7 @@ generate _pkg_descr lbi = concat $
   "/* DO NOT EDIT: This file is automatically generated by Cabal */\n\n" :
   [ concat
     ["/* package ",display pkgid," */\n"
-    ,"#define MIN_VERSION_",display name,"(major1,major2,minor) \\\n"
+    ,"#define MIN_VERSION_",pkgname,"(major1,major2,minor) \\\n"
     ,"  (major1) <  ",major1," || \\\n"
     ,"  (major1) == ",major1," && (major2) <  ",major2," || \\\n"
     ,"  (major1) == ",major1," && (major2) == ",major2," && (minor) <= ",minor
@@ -48,4 +48,8 @@ generate _pkg_descr lbi = concat $
     ]
   | pkgid@(PackageIdentifier name version) <- packageDeps lbi
   , let (major1:major2:minor:_) = map show (versionBranch version ++ repeat 0)
+        pkgname = map fixchar (display name)
   ]
+  where fixchar '-' = '_'
+        fixchar c   = c
+