Fix thread detection with uClibc in timer_time.m4

The timer_time.m4 gl_TIMER_TIME function determines which libraries
need to be linked to get access to the timer function, generally -lrt
for Linux systems. On platforms where threads are used, librt
typically uses thread functions from libpthread.

However, the test to determine whether the platform has thread or not
is incorrect: it assumes that if the C library is uClibc, then threads
are not available. This is actually not true: uClibc has configurable
thread support, and when thread support is available, librt calls
libpthread functions.

This is important when static linking is used, because otherwise only
-lrt is used at link time, which fails because librt calls undefined
thread functions. Both -lrt and -lpthread must be passed.

This problem is fixed by making the uClibc thread detection a bit
smarter, thanks to the usage of the __HAS_NO_THREADS__ macro defined
in <bits/uClibc_config.h>, which itself is included by <features.h>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/m4/timer_time.m4
===================================================================
--- a/m4/timer_time.m4
+++ b/m4/timer_time.m4
@@ -28,7 +28,7 @@
 #include <features.h>
 #ifdef __GNU_LIBRARY__
  #if ((__GLIBC__ == 2 && __GLIBC_MINOR__ >= 2) || (__GLIBC__ > 2)) \
-     && !defined __UCLIBC__
+     && !(__UCLIBC__ && __HAS_NO_THREADS__)
   Thread emulation available
  #endif
 #endif