[test] Test that the exec-stack bit isn't set on libzstd.so

Tests that libzstd.so doesn't have the exec-stack bit set using
readelf. If the stack is marked executable systemd will refuse
to link against zstd. We now test that it isn't set on every PR.

Adds a test for PR #2857
Fixes Issue #2865

tests/libzstd_builds.sh

@@ -23,14 +23,19 @@ mustBeAbsent() {
 
 # default compilation : all features enabled - no zbuff
 $ECHO "testing default library compilation"
-CFLAGS= make -C $DIR/../lib libzstd.a > $INTOVOID
+CFLAGS= make -C $DIR/../lib libzstd libzstd.a > $INTOVOID
 nm $DIR/../lib/libzstd.a | $GREP "\.o" > tmplog
 isPresent "zstd_compress.o"
 isPresent "zstd_decompress.o"
 isPresent "zdict.o"
 isPresent "zstd_v07.o"
 mustBeAbsent "zbuff_compress.o"
-$RM $DIR/../lib/libzstd.a tmplog
+$RM tmplog
+
+# Check that the exec-stack bit isn't set
+readelf -lW $DIR/../lib/libzstd.so | $GREP "GNU_STACK" > tmplog
+mustBeAbsent "RWE"
+$RM $DIR/../lib/libzstd.a $DIR/../lib/libzstd.so* tmplog
 
 # compression disabled => also disable zdict
 $ECHO "testing with compression disabled"