configure: Check for nanosleep in headers as well, not only in libs

On mingw64 with c++11 support, the link libraries do contain a
nanosleep function, while it isn't exposed via the headers. Using
check_func_headers instead of a plain check_func fixes this
misdetection.
Suggested-by: Hendrik Leppkes <h.leppkes@gmail.com>
See: [FFmpeg-devel] [PATCH] fix: 'make' with mingw32
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -4659,7 +4659,7 @@ check_func  mkstemp
 check_func  mmap
 check_func  mprotect
 # Solaris has nanosleep in -lrt, OpenSolaris no longer needs that
-check_func  nanosleep || { check_func nanosleep -lrt && add_extralibs -lrt; }
+check_func_headers time.h nanosleep || { check_func_headers time.h nanosleep -lrt && add_extralibs -lrt; }
 check_func  sched_getaffinity
 check_func  setrlimit
 check_struct "sys/stat.h" "struct stat" st_mtim.tv_nsec -D_BSD_SOURCE