Makefile: improve Jansson-is-missing error handling

Rather than letting the compiler fail due to the Jansson header file
missing, check if it is installed, and, if it is not, provide an
informative error message for installing it or building without it.

Change-Id: I6bf6195e9c27faa6096a3ca41223c41a4da8c6c5
Signed-off-by: jty2 <46021128+jty2@users.noreply.github.com>

Author: jty2

benchmarks/lockhammer/Makefile

@@ -408,6 +408,39 @@ LOCK_DEPS = $(addprefix $(BUILD_DIR)/,$(addsuffix .d,$(addprefix $(LOCK_BASENAME
 LOCK_I = $(addprefix $(BUILD_DIR)/,$(addsuffix .i,$(addprefix $(LOCK_BASENAME).,$(TEST_TARGETS))))
 
 
+# ---------------------
+# early dependency checking
+
+define no_libjansson_message
+
+
+Jansson (for JSON result output) does not appear to be installed.
+
+To build without JSON output, rerun make with NO_JSON=1, e.g.:
+
+make $(MAKECMDGOALS) NO_JSON=1
+
+Or install Jansson development headers, perhaps using:
+
+sudo apt install libjansson-dev
+
+and then run 'make clean' before trying to rebuild
+endef
+
+# This variable is only for the header existience check.  Modify this and
+# CFLAGS/LDFLAGS if Jansson is installed in another location.
+
+JANSSON_HEADER_FILE=/usr/include/jansson.h
+
+ifneq ($(NO_JSON),1)
+ifeq ($(filter $(ALL_CLEAN_TARGETS),$(MAKECMDGOALS)),)
+ifeq (,$(wildcard $(JANSSON_HEADER_FILE)))
+$(error $(no_libjansson_message))
+endif
+endif
+endif
+
+
 # ---------------------
 # info for debugging make