Add symbol tests for apple

closes #21

test_symbols.sh

@@ -2,5 +2,15 @@
 
 set -euo pipefail
 
+# ./test_symbols.sh <library> <expected exported symbols file> <allowed imported symbols file>
+
 diff -u <(sort < "$2") <(nm "$1" | grep " T " | cut -f3 -d " " | sort)
-nm "$1" | grep " U " | (! grep -Pv 'abort|free|malloc|mem[a-z]*|__ashlti3|__stack_chk_[a-z]*|__tls_get_addr|_GLOBAL_OFFSET_TABLE_')
+ec=0
+for symbol in $(nm "$1" | grep " U " | sed 's/ U //') ; do
+    if ! grep "$symbol" "$3" > /dev/null ; then
+        echo "Imported symbol $symbol not present in $3"
+        ec=1
+    fi
+done
+
+exit $ec