i386-gen: preserve fp control word in gen_cvt_ftoi

- Use runtime function for conversion
- Also initialize fp with tcc -run on windows

This fixes a bug where
  double x = 1.0;
  double y = 1.0000000000000001;
  double z = x < y ? 0 : sqrt (x*x - y*y);
caused a bad sqrt because rounding precision for the x < y comparison
was different to the one used within the sqrt function.

This also fixes a bug where
  printf("%d, %d", (int)pow(10, 2), (int)pow(10, 2));
would print
  100, 99

Unrelated:
  win32: document relative include & lib lookup
  win32: normalize_slashes: do not mirror silly gcc behavior
  This reverts part of commit 8a81f9e103
  winapi: add missing WINAPI decl. for some functions

win32/lib/crt1.c

@@ -31,4 +31,10 @@ int _start(void)
     exit(ret);
 }
 
+int _runmain(int argc, char **argv)
+{
+    _controlfp(0x10000, 0x30000);
+    return main(argc, argv, NULL);
+}
+
 // =============================================

win32/lib/wincrt1.c

@@ -59,6 +59,6 @@ int _runwinmain(int argc, char **argv)
         szCmd = "";
     else if (szCmd > p && szCmd[-1] == '\"')
         --szCmd;
+    _controlfp(0x10000, 0x30000);
     return WinMain(GetModuleHandle(NULL), NULL, szCmd, SW_SHOWDEFAULT);
 }
-