Replace WINDOWS with WIN32 in CMakeLists.txt

Fixes Windows specific code in CMake files by using
the proper platform variable for Windows.

This is a slightly modified version of @cedricp changes
proposed in NatronGitHub#1040.

Author: acolwell

App/CMakeLists.txt

@@ -18,7 +18,7 @@
 # ***** END LICENSE BLOCK *****
 
 set(Natron_SOURCES NatronApp_main.cpp)
-if(WINDOWS)
+if(WIN32)
     list(APPEND Natron_SOURCES ../Natron.rc)
 endif()
 add_executable(Natron ${Natron_SOURCES})

Renderer/CMakeLists.txt

@@ -18,7 +18,7 @@
 # ***** END LICENSE BLOCK *****
 
 set(NatronRenderer_SOURCES NatronRenderer_main.cpp)
-if(WINDOWS)
+if(WIN32)
     list(APPEND NatronRenderer_SOURCES ../Natron.rc)
 endif()
 add_executable(NatronRenderer ${NatronRenderer_SOURCES})

libs/gflags/CMakeLists.txt

@@ -30,7 +30,7 @@ set(gflags_SOURCES
     src/gflags_completions.cc
     src/gflags_reporting.cc
 )
-if(WINDOWS)
+if(WIN32)
     set(gflags_HEADERS ${gflags_HEADERS} src/windows_port.h)
     set(gflags_SOURCES ${gflags_SOURCES} src/windows_port.cc)
 endif()