build: use BUILT_SOURCES for the generated source files.

This avoids possible race conditions on heavily multi-core systems.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
diff --git a/src/Makefile.am b/src/Makefile.am
index 36f95ee..6540098 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -17,6 +17,8 @@
 			 conv.c application.c rbtree.c \
 			 crc8gen.c crc16gen.c crc32gen.c crc64gen.c
 
+BUILT_SOURCES = crc8gen.c crc16gen.c crc32gen.c crc64gen.c
+
 if ENABLE_PLUGIN
 libosmocore_la_SOURCES += plugin.c
 libosmocore_la_LDFLAGS = -version-info $(LIBVERSION) $(LIBRARY_DL)