fix windows/amd64 build with newest mingw-w64

R=alex.brainman, golang-dev
CC=golang-dev
https://golang.org/cl/4968048

include/libc.h

@@ -112,6 +112,7 @@ extern	void	sysfatal(char*, ...);
 #define notejmp		p9notejmp
 #define jmp_buf		p9jmp_buf
 #define pow10		p9pow10
+#undef  strtod
 #define strtod		fmtstrtod
 #define charstod	fmtcharstod
 #endif
@@ -306,6 +307,7 @@ extern int nanosleep(const struct timespec *rqtp, struct timespec *rmtp);
 extern int fork(void);
 extern int pread(int fd, void *buf, int n, int off);
 extern int pwrite(int fd, void *buf, int n, int off);
+#undef  lseek
 #define lseek(fd, n, base) _lseeki64(fd, n, base)
 #define mkdir(path, perm) mkdir(path)
 #define pipe(fd) _pipe(fd, 512, O_BINARY)