Fix -j race on libsashidown.a; cst-agent-update keeps the install prefix

Makefile

@@ -103,7 +103,7 @@ TESTS = \
 
 TEST_LIBS = -lm $(CURL_LIBS) $(PTHREAD_LIBS)
 
-.PHONY: all clean check demo sashidown-lib cmake-gcc cmake-clang asan install uninstall
+.PHONY: all clean check demo sashidown-lib FORCE cmake-gcc cmake-clang asan install uninstall
 
 all: sashidown-lib cst-agent
 
@@ -116,9 +116,15 @@ cmake-clang:
 	cmake -B build-clang -DCMAKE_C_COMPILER=clang -DCST_BUILD_TUI=ON
 	cmake --build build-clang
 
-sashidown-lib:
+sashidown-lib: $(SD_DIR)/libsashidown.a
+
+# The sub-make decides whether the archive is stale; FORCE only makes sure it
+# is asked, and -j builds wait for it before linking.
+$(SD_DIR)/libsashidown.a: FORCE
 	$(MAKE) -C $(SD_DIR) libsashidown.a
 
+FORCE:
+
 cst-agent: $(OBJ) $(SD_DIR)/libsashidown.a
 	$(CC) $(CFLAGS) -o $@ $(OBJ) $(LDFLAGS) $(LDLIBS)