build: let autoconf detect system extensions

Move to AC_USE_SYSTEM_EXTENSIONS for automation of detecting supported source
extensions, such as:

* -D_DEFAULT_SOURCE
* -D_BSD_SOURCE
* -D_POSIX_SOURCE

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>

diff --git a/configure.ac b/configure.ac
index dc4b540..705be62 100644
--- a/configure.ac
+++ b/configure.ac

@@ -27,6 +27,7 @@
 
 AC_PROG_CXX
 AC_PROG_CC
+AC_USE_SYSTEM_EXTENSIONS
 LT_INIT()
 PKG_INSTALLDIR()
 
@@ -243,9 +244,6 @@
               [enable_defaultflags=yes])
 AS_IF([test "x$enable_defaultflags" = "xyes"],
       [
-      ADD_PREPROC_FLAG([-D_DEFAULT_SOURCE])
-      ADD_PREPROC_FLAG([-D_BSD_SOURCE])
-      ADD_PREPROC_FLAG([-D_POSIX_SOURCE])
       AS_IF([test "x$enable_debug" = "xno"],
             [AX_ADD_FORTIFY_SOURCE])
       ADD_COMPILER_FLAG([-std=c99])

diff --git a/src/tss2-esys/esys_crypto.c b/src/tss2-esys/esys_crypto.c
index d62366e..aef3e50 100644
--- a/src/tss2-esys/esys_crypto.c
+++ b/src/tss2-esys/esys_crypto.c

@@ -4,8 +4,6 @@
  * All rights reserved.
  ******************************************************************************/
 
-#define _GNU_SOURCE
-
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif

diff --git a/src/tss2-esys/esys_crypto_gcrypt.c b/src/tss2-esys/esys_crypto_gcrypt.c
index 55904a1..0ebe88a 100644
--- a/src/tss2-esys/esys_crypto_gcrypt.c
+++ b/src/tss2-esys/esys_crypto_gcrypt.c

@@ -4,8 +4,6 @@
  * All rights reserved.
  ******************************************************************************/
 
-#define _GNU_SOURCE
-
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif

diff --git a/src/tss2-esys/esys_crypto_ossl.c b/src/tss2-esys/esys_crypto_ossl.c
index a72a4f2..1245019 100644
--- a/src/tss2-esys/esys_crypto_ossl.c
+++ b/src/tss2-esys/esys_crypto_ossl.c

@@ -4,8 +4,6 @@
  * All rights reserved.
  ******************************************************************************/
 
-#define _GNU_SOURCE
-
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif

diff --git a/test/integration/esys-unseal-password-auth.int.c b/test/integration/esys-unseal-password-auth.int.c
index db583dc..dd1647b 100644
--- a/test/integration/esys-unseal-password-auth.int.c
+++ b/test/integration/esys-unseal-password-auth.int.c

@@ -4,8 +4,6 @@
  * All rights reserved.
  *******************************************************************************/
 
-#define _GNU_SOURCE
-
 #ifdef HAVE_CONFIG_H
 #include <config.h>
 #endif