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.
Signed-off-by: NMartin Storsjö <martin@martin.st>

configure

@@ -4046,7 +4046,7 @@ 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_func  strerror_r