Fixes

./compat/getentropy_linux.c:390:4: warning: implicit declaration of function 'getsid' [-Wimplicit-function-declaration]
    HX((pid = getsid(pid)) == -1, pid);
    ^
./compat/getentropy_linux.c:392:4: warning: implicit declaration of function 'getpgid' [-Wimplicit-function-declaration]
    HX((pid = getpgid(0)) == -1, pid);
    ^

--- configure
+++ configure
@@ -4081,7 +4081,7 @@
 
 
 case "$host_os" in
-  linux* ) CFLAGS="$CFLAGS -D_BSD_SOURCE -D_DEFAULT_SOURCE"
+  linux* ) CFLAGS="$CFLAGS -D_GNU_SOURCE"
   ;;
   solaris* ) CFLAGS="$CFLAGS -D__EXTENSIONS__"  # for strdup() from <string.h>
   ;;