nettle: fix build on macos xcode 12

compiler warns that exit() isn't defined so checks for build system
compiler fail.

include <stdlib.h> to define exit()

Tested under macos Catalina & Big Sur

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

package/libs/nettle/patches/100-portability.patch

@@ -0,0 +1,34 @@
+--- a/configure
++++ b/configure
+@@ -4635,6 +4635,7 @@ $as_echo_n "checking build system compil
+ # remove anything that might look like compiler output to our "||" expression
+ rm -f conftest* a.out b.out a.exe a_out.exe
+ cat >conftest.c <<EOF
++#include <stdlib.h>
+ int
+ main ()
+ {
+@@ -4667,6 +4668,7 @@ $as_echo_n "checking build system compil
+ # remove anything that might look like compiler output to our "||" expression
+ rm -f conftest* a.out b.out a.exe a_out.exe
+ cat >conftest.c <<EOF
++#include <stdlib.h>
+ int
+ main ()
+ {
+@@ -4703,6 +4705,7 @@ $as_echo_n "checking build system compil
+ # remove anything that might look like compiler output to our "||" expression
+ rm -f conftest* a.out b.out a.exe a_out.exe
+ cat >conftest.c <<EOF
++#include <stdlib.h>
+ int
+ main ()
+ {
+@@ -4753,6 +4756,7 @@ else
+   gmp_cv_prog_exeext_for_build="$EXEEXT"
+ else
+   cat >conftest.c <<EOF
++#include <stdlib.h>
+ int
+ main ()
+ {