Some initial changes (currently for libpbd only) to help a new contributer who's trying to build with MSVC2017

These changes compile okay for me (using VS2019) although they wouldn't link to my older-built libraries. Hopefully he'll be okay if he builds everything with the same compiler.

msvc_extra_headers/ardourext/float_cast.h.input

@@ -40,8 +40,9 @@
 
 	#include	<math.h>
 
-	/*	Win32 doesn't seem to have these functions.
-	**	Therefore implement inline versions of these functions here.
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
+/*	Win32 doesn't seem to have these functions.
+	*	Therefore implement inline versions of these functions here.
 	*/
 
 	__inline long int
@@ -115,6 +116,8 @@
 			
 		return intgr ;
 	}
-#endif
+#endif  // _MSC_VER && _MSC_VER < 1900
+
+#endif  // _WIN32 || WIN32
 
 #endif  // __FLOAT_CAST_H__

msvc_extra_headers/ardourext/misc.h.input

@@ -19,9 +19,19 @@
 #ifndef __ardour_msvc_extensions_h__
 #define __ardour_msvc_extensions_h__
 
-#ifndef _WIN32_WINNT
-#define _WIN32_WINNT 0x0500
+#ifdef WINVER
+#undef WINVER
 #endif
+#if (defined(_MSC_VER) && (_MSC_VER < 1900)) /* 1900 is an arbitrary value, corresponding to VS2019 (it might need to be lowered) */
+#define WINVER 0x0500
+#else
+#define WINVER 0x0601
+#endif
+#ifdef _WIN32_WINNT
+#undef _WIN32_WINNT
+#endif
+#define _WIN32_WINNT WINVER
+
 #ifndef _CPP_VECTOR
 #define _CPP_VECTOR  1
 #endif
@@ -208,8 +218,10 @@ inline int64_t abs(int64_t val) throw()
 #endif
 
 // fmin() and fmax()
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
 #define fmin(a, b) min((double)a, (double)b)
 #define fmax(a, b) max((double)a, (double)b)
+#endif
 
 // approximate POSIX pipe()
 #define pipe(handles) _pipe(handles, 4096, _O_BINARY)

msvc_extra_headers/ardourext/ptw32/pthread.h.input

@@ -308,11 +308,13 @@ enum {
 #endif
 
 #ifndef HAVE_STRUCT_TIMESPEC
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
 #define HAVE_STRUCT_TIMESPEC 1
 struct timespec {
         long tv_sec;
         long tv_nsec;
 };
+#endif /* _MSC_VER */
 #endif /* HAVE_STRUCT_TIMESPEC */
 
 #ifndef SIG_BLOCK