test/unit/Makefile: run the test in build dir

In case that eg. LD_LIBRARY_PATH is set by direnv, it is better to run
in build dir as a working directory.

+ append the passed $(LIBDIR) to $LD_LIBRARY_PATH rather than replacing

Author: MartinPulec

test/unit/Makefile

@@ -2,7 +2,8 @@ BUILD_DIR ?= .
 LIBDIR ?= ../../build
 
 all: run_tests
-	LD_LIBRARY_PATH=$(LIBDIR) $(BUILD_DIR)/$<
+	cd $(BUILD_DIR);\
+	LD_LIBRARY_PATH=$${LD_LIBRARY_PATH:+$$LD_LIBRARY_PATH:}$(LIBDIR) ./$<
 
 run_tests: $(wildcard *.c)
 	$(CC) -g -I../.. $^ -L$(LIBDIR) -lgpujpeg -lcudart -o $(BUILD_DIR)/$@