use getrandom() on recent linux distros

Instead of using the internal DRBG-based RNG, use the `getrandom()` system
call where available.
Keep the existing version as fall-back for other systems that don't provide
that system call.

... and for cases where auto-detection fails or the user simply wants to
use the internal RNG a new configure-switch is provided.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

configure.ac

@@ -41,6 +41,8 @@ AC_ARG_ENABLE([tls],
     [AS_HELP_STRING([--disable-tls], [disable TLS support])])
 AC_ARG_ENABLE([cares],
     [AS_HELP_STRING([--enable-cares], [use c-ares for DNS resolution])])
+AC_ARG_ENABLE([getrandom],
+    [AS_HELP_STRING([--disable-getrandom], [disable usage of the getrandom() system call])])
 
 AC_ARG_ENABLE([fuzzing],
     [AS_HELP_STRING([--enable-fuzzing], [turn on fuzzing test])],
@@ -96,6 +98,12 @@ elif test "x$enable_tls" != xno; then
         )])
 fi
 
+AC_CHECK_FUNCS([getrandom], [], [enable_getrandom=no])
+if test "$enable_getrandom" = "no"; then
+    AC_DEFINE(DONT_USE_GETRANDOM)
+    AC_MSG_NOTICE([libstrophe will not use the getrandom() system call])
+fi
+
 with_parser=""
 if test "x$with_libxml2" != xyes; then
     PKG_CHECK_MODULES([expat], [expat >= 2.0.0],