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

libtcc.c

@@ -78,21 +78,13 @@ ST_FUNC void asm_global_instr(void)
 #endif
 
 /********************************************************/
-
 #ifdef _WIN32
-// GCC appears to use '/' for relative paths and '\\' for absolute paths on Windows
 static char *normalize_slashes(char *path)
 {
     char *p;
-    if (path[1] == ':') {
-        for (p = path+2; *p; ++p)
-            if (*p == '/')
-                *p = '\\';
-    } else {
-        for (p = path; *p; ++p)
-            if (*p == '\\')
-                *p = '/';
-    }
+    for (p = path; *p; ++p)
+        if (*p == '\\')
+            *p = '/';
     return path;
 }
 
@@ -1036,6 +1028,7 @@ LIBTCCAPI TCCState *tcc_new(void)
 #ifdef TCC_TARGET_I386
     s->seg_size = 32;
 #endif
+    s->runtime_main = "main";
     return s;
 }