Simplify code via AC_USE_SYSTEM_EXTENSIONS

`AC_USE_SYSTEM_EXTENSIONS` takes care of defining feature_test_macros(7)
thereby allowing us to remove a few manual `#define`s

cbits/dirUtils.c

@@ -4,17 +4,6 @@
  * Directory Runtime Support
  */
 
-/* needed only for solaris2_HOST_OS */
-#ifdef __GLASGOW_HASKELL__
-#include "ghcconfig.h"
-#endif
-
-// The following is required on Solaris to force the POSIX versions of
-// the various _r functions instead of the Solaris versions.
-#ifdef solaris2_HOST_OS
-#define _POSIX_PTHREAD_SEMANTICS
-#endif
-
 #include "HsUnix.h"
 
 /*

cbits/execvpe.c

@@ -11,10 +11,6 @@
 
 #include "HsUnixConfig.h"
 
-#if HAVE_EXECVPE
-# define _GNU_SOURCE
-#endif
-
 #include <errno.h>
 #include <sys/types.h>
 #if HAVE_SYS_WAIT_H

configure.ac

 AC_INIT([Haskell unix package], [2.0], [libraries@haskell.org], [unix])
+AC_PREREQ([2.60])
 
 # Safety check: Ensure that we are in the correct source directory.
 AC_CONFIG_SRCDIR([include/HsUnix.h])
 
+AC_USE_SYSTEM_EXTENSIONS
+
 AC_ARG_WITH([cc],
             [C compiler],
             [CC=$withval])

include/HsUnix.h

@@ -19,10 +19,6 @@
 #undef PACKAGE_TARNAME
 #undef PACKAGE_VERSION
 
-#ifdef solaris2_HOST_OS
-#define _POSIX_PTHREAD_SEMANTICS
-#endif
-
 #include <stdlib.h>
 #include <stdio.h>
 

include/execvpe.h

@@ -14,7 +14,6 @@ __hsunix_execvpe(const char *name, char *const argv[], char *const envp[]);
 #ifndef HSUNIX_EXECVPE_H_NO_COMPAT
 #include "HsUnixConfig.h"
 #if HAVE_EXECVPE
-# define _GNU_SOURCE
 # include <unistd.h>
 extern int
 execvpe(const char *name, char *const argv[], char *const envp[]);