diff --git a/CMakeLists.txt b/CMakeLists.txt
index acb8b44..ae24799 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,9 +1,9 @@
 cmake_minimum_required(VERSION 2.8.4 FATAL_ERROR)
 
 if (WIN32 AND NOT CYGWIN)
-  set (WINDOWS 1)
+  set (OS_WINDOWS 1)
 else ()
-  set (WINDOWS 0)
+  set (OS_WINDOWS 0)
 endif ()
 
 # ----------------------------------------------------------------------------
@@ -34,7 +34,7 @@ version_numbers (
 # configure options
 option (BUILD_SHARED_LIBS "Request build of shared libraries." OFF)
 
-if (WINDOWS AND BUILD_SHARED_LIBS)
+if (OS_WINDOWS AND BUILD_SHARED_LIBS)
   set (GFLAGS_IS_A_DLL 1)
 else ()
   set (GFLAGS_IS_A_DLL 0)
@@ -184,7 +184,7 @@ set (GFLAGS_SRCS
   "gflags_completions.cc"
 )
 
-if (WINDOWS)
+if (OS_WINDOWS)
   list (APPEND PRIVATE_HDRS "windows_port.h")
   list (APPEND GFLAGS_SRCS  "windows_port.cc")
 endif ()
@@ -226,7 +226,7 @@ endif ()
 
 # ----------------------------------------------------------------------------
 # installation
-if (WINDOWS)
+if (OS_WINDOWS)
   set (RUNTIME_INSTALL_DIR Bin)
   set (LIBRARY_INSTALL_DIR Lib)
   set (INCLUDE_INSTALL_DIR Include)
diff --git a/src/config.h.in b/src/config.h.in
index 24b513e..3f94663 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -5,6 +5,9 @@
 // ---------------------------------------------------------------------------
 // System checks
 
+// Define if you build this library for a MS Windows OS.
+#cmakedefine OS_WINDOWS
+
 // Define if you have the <stdint.h> header file.
 #cmakedefine HAVE_STDINT_H
 
@@ -73,7 +76,7 @@
 // ---------------------------------------------------------------------------
 // Path separator
 #ifndef PATH_SEPARATOR
-#  if _WIN32
+#  ifdef OS_WINDOWS
 #    define PATH_SEPARATOR  '\\'
 #  else
 #    define PATH_SEPARATOR  '/'
@@ -101,7 +104,7 @@
 #  define GFLAGS_DLL_DEFINE_FLAG GFLAGS_DLL_DECL
 #endif
 
-#ifdef _WIN32
+#ifdef OS_WINDOWS
 // The unittests import the symbols of the shared gflags library
 #  if GFLAGS_IS_A_DLL && defined(_MSC_VER)
 #    define GFLAGS_DLL_DECL_FOR_UNITTESTS __declspec(dllimport)
diff --git a/src/gflags.cc b/src/gflags.cc
index bd1b130..b314f08 100644
--- a/src/gflags.cc
+++ b/src/gflags.cc
@@ -1494,7 +1494,7 @@ const char* ProgramInvocationName() {             // like the GNU libc fn
 }
 const char* ProgramInvocationShortName() {        // like the GNU libc fn
   const char* slash = strrchr(argv0, '/');
-#ifdef _WINDOWS
+#ifdef OS_WINDOWS
   if (!slash)  slash = strrchr(argv0, '\\');
 #endif
   return slash ? slash + 1 : argv0;
diff --git a/src/mutex.h b/src/mutex.h
index 3e2f28e..0bdd9d5 100644
--- a/src/mutex.h
+++ b/src/mutex.h
@@ -110,7 +110,7 @@
 
 #if defined(NO_THREADS)
   typedef int MutexType;        // to keep a lock-count
-#elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__)
+#elif defined(OS_WINDOWS)
 # ifndef WIN32_LEAN_AND_MEAN
 #   define WIN32_LEAN_AND_MEAN  // We only need minimal includes
 # endif
@@ -227,7 +227,7 @@ bool Mutex::TryLock()      { if (mutex_) return false; Lock(); return true; }
 void Mutex::ReaderLock()   { assert(++mutex_ > 0); }
 void Mutex::ReaderUnlock() { assert(mutex_-- > 0); }
 
-#elif defined(_WIN32) || defined(__CYGWIN32__) || defined(__CYGWIN64__)
+#elif defined(OS_WINDOWS)
 
 Mutex::Mutex() : destroy_(true) {
   InitializeCriticalSection(&mutex_);
diff --git a/src/util.h b/src/util.h
index 6991c37..de94091 100644
--- a/src/util.h
+++ b/src/util.h
@@ -65,7 +65,7 @@ extern GFLAGS_DLL_DECL void (*gflags_exitfunc)(int);
 #elif defined(HAVE_STRTOQ)
 #  define strto64  strtoq
 #  define strtou64 strtouq
-#elif defined(_WIN32) && !defined(__MINGW32__) && !defined(__MINGW64__)
+#elif defined(OS_WINDOWS)
 #  define strto64  _strtoi64
 #  define strtou64 _strtoui64
 // Neither strtoll nor strtoq are defined.  I hope strtol works!
diff --git a/src/windows_port.h b/src/windows_port.h
index 246f715..c8ff24f 100644
--- a/src/windows_port.h
+++ b/src/windows_port.h
@@ -112,7 +112,7 @@ inline void setenv(const char* name, const char* value, int) {
 #define PRId64  "I64d"
 #define PRIu64  "I64u"
 
-#ifndef __MINGW32__
+#if !defined(__MINGW32__) && !defined(__MINGW64__)
 #define strtoq   _strtoi64
 #define strtouq  _strtoui64
 #define strtoll  _strtoi64