omniorb/thread: use proper autoconf macros for header inclusion

src/lib/omnithread/posix.cc uses sleep() and usleep(), defined in
<unistd.h> on numerous platforms, and struct timeval/gettimeofday(),
defined in <sys/time.h> on various platforms.

Since those header files are not available on all platforms, posix.cc
currently uses the following condition:

#if (defined(__GLIBC__) && __GLIBC__ >= 2) || defined(__SCO_VERSION__) || defined(__aix__) || defined (__cygwin__) || defined(__darwin__) || defined(__macos__)

Unfortunately, this falls short on Linux systems based on the musl C
library. Indeed, the musl C library does not define the __GLIBC__
symbol, but does have the sleep()/usleep() definitions in <unistd.h>,
and the struct timeval/gettimeofday() definitions in <sys/time.h>,
like any Linux system. Also, the musl C library does not define any
constant like __MUSL__ to distinguish it.

Due to this, on musl based systems, <unistd.h> and <sys/time.h> are
not included, causing a build failure:

posix.cc:864:22: error: '::sleep' has not been declared
       while ((secs = ::sleep(secs))) ;
                      ^
posix.cc:866:43: error: 'usleep' was not declared in this scope
  usleep(secs * 1000000 + (nanosecs / 1000));
                                           ^
posix.cc: In static member function 'static void omni_thread::get_time(long unsigned int*, long unsigned int*, long unsigned int, long unsigned int)':
posix.cc:904:20: error: aggregate 'omni_thread::get_time(long unsigned int*, long unsigned int*, long unsigned int, long unsigned int)::timeval tv' has incomplete type and cannot be defined
     struct timeval tv;
                    ^
posix.cc:905:24: error: 'gettimeofday' was not declared in this scope
     gettimeofday(&tv, 0); 

It turns out that the configure.ac already checks for the availability
of <unistd.h> and <sys/time.h>. So all what this patch does is use the
C defines generated by the configure script to decide whether
<unistd.h> and <sys/time.h> can be included or not.

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

Index: b/src/lib/omnithread/posix.cc
===================================================================
--- a/src/lib/omnithread/posix.cc
+++ b/src/lib/omnithread/posix.cc
@@ -78,10 +78,14 @@
 #include <errno.h>
 #include <time.h>
 #include <omnithread.h>
+#include <omniORB4/acconfig.h>
 
-#if (defined(__GLIBC__) && __GLIBC__ >= 2) || defined(__SCO_VERSION__) || defined(__aix__) || defined (__cygwin__) || defined(__darwin__) || defined(__macos__)
 // typedef of struct timeval and gettimeofday();
+#if defined(HAVE_SYS_TIME_H)
 #include <sys/time.h>
+#endif
+
+#if defined(HAVE_UNISTD_H)
 #include <unistd.h>
 #endif