Cleaner solution for echoing the SOURCES variable.

src/Makefile.am

@@ -20,3 +20,5 @@ libzzuf_la_CFLAGS = -DLIBZZUF
 libzzuf_la_LDFLAGS = -avoid-version -no-undefined $(DLL_LDFLAGS)
 libzzuf_la_LIBADD = $(GETOPT_LIBS) $(DL_LIBS) $(MATH_LIBS) $(WINSOCK2_LIBS)
 
+echo-sources: ; echo $(SOURCES)
+

test/Makefile.am

@@ -6,3 +6,5 @@ noinst_PROGRAMS = zzcat zzero zznop bug-overflow bug-memory bug-div0
 TESTS = check-build check-md5 check-overflow check-memory check-div0 \
         check-rng check-utils
 
+echo-sources: ; echo $(SOURCES)
+

test/check-build

@@ -29,9 +29,7 @@ fi
 failure=0
 for dir in src test; do
   pushd ../$dir >/dev/null
-  # Dirty hack to print the $(SOURCES) variable
-  for x in $(make -s ID SHELL='echo @@$(SOURCES)@@' | tr -d '\n' | sed 's/.*@@\([^@]*\)@@.*/\1/');
-  do
+  for x in $(make echo-sources); do
     if grep '[[:space:]]$' "$x" >/dev/null 2>&1; then
       echo "error: $dir/$x contains trailing spaces"
       failure=1