Makefile: fix j for clang-tidy calls

Makefile

@@ -75,6 +75,9 @@ ARGS := $(wordlist 2,$(words $(MAKECMDGOALS)),$(MAKECMDGOALS))
 MAKE_PID := $(shell echo $$PPID)
 j := $(shell ps T | sed -n 's/.*$(MAKE_PID).*$(MAKE).* \(-j\|--jobs\) *\([0-9][0-9]*\).*/\2/p')
 
+# Default j for clang-tidy
+j_clang_tidy := $(or $(j),4)
+
 NINJA_BIN := ninja
 ifndef NO_NINJA_BUILD
 	NINJA_BUILD := $(shell $(NINJA_BIN) --version 2>/dev/null)
@@ -429,16 +432,16 @@ px4_sitl_default-clang:
 	@$(PX4_MAKE) -C "$(SRC_DIR)"/build/px4_sitl_default-clang
 
 clang-tidy: px4_sitl_default-clang
-	@cd "$(SRC_DIR)"/build/px4_sitl_default-clang && "$(SRC_DIR)"/Tools/run-clang-tidy.py -header-filter=".*\.hpp" -j$(j) -p .
+	@cd "$(SRC_DIR)"/build/px4_sitl_default-clang && "$(SRC_DIR)"/Tools/run-clang-tidy.py -header-filter=".*\.hpp" -j$(j_clang_tidy) -p .
 
 # to automatically fix a single check at a time, eg modernize-redundant-void-arg
 #  % run-clang-tidy-4.0.py -fix -j4 -checks=-\*,modernize-redundant-void-arg -p .
 clang-tidy-fix: px4_sitl_default-clang
-	@cd "$(SRC_DIR)"/build/px4_sitl_default-clang && "$(SRC_DIR)"/Tools/run-clang-tidy.py -header-filter=".*\.hpp" -j$(j) -fix -p .
+	@cd "$(SRC_DIR)"/build/px4_sitl_default-clang && "$(SRC_DIR)"/Tools/run-clang-tidy.py -header-filter=".*\.hpp" -j$(j_clang_tidy) -fix -p .
 
 # modified version of run-clang-tidy.py to return error codes and only output relevant results
 clang-tidy-quiet: px4_sitl_default-clang
-	@cd "$(SRC_DIR)"/build/px4_sitl_default-clang && "$(SRC_DIR)"/Tools/run-clang-tidy.py -header-filter=".*\.hpp" -j$(j) -p .
+	@cd "$(SRC_DIR)"/build/px4_sitl_default-clang && "$(SRC_DIR)"/Tools/run-clang-tidy.py -header-filter=".*\.hpp" -j$(j_clang_tidy) -p .
 
 # TODO: Fix cppcheck errors then try --enable=warning,performance,portability,style,unusedFunction or --enable=all
 cppcheck: px4_sitl_default