windows support: fix compilation with pthreads

This caused errors like:

    core/mplayer.c:4308:5: error: implicit declaration of function 'pthread_win32_thread_detach_np' [-Werror=implicit-function-declaration]

It turns out a pthread.h include was missing. It's not clear why this
used to work (or rather, why it happens only sometimes). Possibly some
libraries or system headers recursively include pthread.h under certain
circumstances or configurations.

Fix missing quoting in configure, which led to broken terminal output.

Closes #6.

configure

@@ -1490,7 +1490,7 @@ cc_check $_ld_tmp -DPTW32_STATIC_LIB && (tmp_run || test "$_ld_static") && _ld_p
 fi
 fi
 if test "$_pthreads" = yes ; then
-  test $_ld_pthread && res_comment="using $_ld_pthread"
+  test "$_ld_pthread" && res_comment="using $_ld_pthread"
   def_pthreads='#define HAVE_PTHREADS 1'
   def_threads='#define HAVE_THREADS 1'
   extra_cflags="$extra_cflags $THREAD_CFLAGS"

core/mplayer.c

@@ -22,6 +22,10 @@
 #include <math.h>
 #include <assert.h>
 
+#ifdef PTW32_STATIC_LIB
+#include <pthread.h>
+#endif
+
 #include <libavutil/intreadwrite.h>
 #include <libavutil/attributes.h>