Use proper cmake detection of features.

This attempts to organize all needed items and places appropriate defines
in a single location. Redundant comparisons/defines were removed when noticed.

A few caveats, however:

* This may be better targeted towards the 5_1_0 branch instead of master right now.
* Cmake will run a little slower on configuration/generation now. That's due to the sanity checks it runs.
* There are some more checks to be added later, but this should be a solid start.

Thanks to the [libical](https://github.com/libical/libical) project for inspiration.

src/archutils/Win32/CrashHandlerChild.cpp

@@ -7,7 +7,9 @@
 #include <commctrl.h>
 #include "archutils/Win32/ddk/dbghelp.h"
 #include <io.h>
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 #include <shellapi.h>
 
 #include "arch/ArchHooks/ArchHooks.h"

src/archutils/Win32/arch_setup.h

@@ -24,8 +24,6 @@
 #pragma warning (disable : 4005) // macro redefinitions (ARRAYSIZE)
 #endif
 
-#define snprintf _snprintf // Unsure if this goes with __MINGW32__ right now.
-
 /*
 The following warnings are disabled in all builds.
 Enable them in the project file at your peril (or if you feel like
@@ -82,7 +80,6 @@ C4355: 'this' : used in base member initializer list
 
 #endif
 
-#include <direct.h> // has stuff that should be in unistd.h
 #include <wchar.h> // needs to be included before our fixes below
 
 #define lstat stat
@@ -90,9 +87,6 @@ C4355: 'this' : used in base member initializer list
 #define isnan _isnan
 #define isfinite _finite
 
-// mkdir is missing the mode arg
-#define mkdir(p,m) mkdir(p)
-
 typedef time_t time_t;
 struct tm;
 struct tm *my_localtime_r( const time_t *timep, struct tm *result );
@@ -135,43 +129,12 @@ int64_t llabs( int64_t i ) { return i >= 0 ? i : -i; }
 #undef max
 #define NOMINMAX // make sure Windows doesn't try to define this
 
-// Windows is missing some basic math functions:
-// But MinGW isn't.
-#if !defined(__MINGW32__) && (!defined(_MSC_VER) || _MSC_VER < 1700) // cstdint and truncf were first implemented in Visual Studio 2012
-#define NEED_TRUNCF
-#define MISSING_STDINT_H
-#endif
-
-// MinGW provides us with this function already
-
-#if !defined(__MINGW32__) && (!defined(_MSC_VER) || _MSC_VER < 1800) // lrintf, roundf and strtof were first implemented in Visual Studio 2013
-#define NEED_ROUNDF
-#define NEED_STRTOF
-inline long int lrintf( float f )
-{
-	int retval;
-
-	_asm fld f;
-	_asm fistp retval;
-
-	return retval;
-}
-#endif
-
-// For RageLog.
-#define HAVE_VERSION_INFO
-
 /* We implement the crash handler interface (though that interface isn't
  * completely uniform across platforms yet). */
 #if !defined(SMPACKAGE)
 #define CRASH_HANDLER
 #endif
 
-// autoconf does this for us
-#if !defined(__MINGW32__)
-#define ENDIAN_LITTLE
-#endif
-
 #if defined(__GNUC__) // It might be MinGW or Cygwin(?)
 #include "archutils/Common/gcc_byte_swaps.h"
 #elif defined(_MSC_VER) && (_MSC_VER >= 1310) // Byte swap functions were first implemented in Visual Studio .NET 2003