chore: move test-specific defines from CXXFLAGS to CPPFLAGS

This ensures that preprocessor flags are managed separately from
compiler flags, avoiding confusion with user-provided CXXOPTS.

Co-authored-by: Oliver Stöneberg <firewave@users.noreply.github.com>

Author: jcfr

Makefile

@@ -1,16 +1,17 @@
 all:	testrunner simplecpp
 
+CPPFLAGS ?=
 CXXFLAGS = -Wall -Wextra -pedantic -Wcast-qual -Wfloat-equal -Wmissing-declarations -Wmissing-format-attribute -Wredundant-decls -Wundef -Wno-multichar -Wold-style-cast -std=c++11 -g $(CXXOPTS)
 LDFLAGS = -g $(LDOPTS)
 
-# Define test source dir macro for compilation
-TEST_DEFINES = -DSIMPLECPP_TEST_SOURCE_DIR=\"$(CURDIR)\"
+# Define test source dir macro for compilation (preprocessor flags)
+TEST_CPPFLAGS = -DSIMPLECPP_TEST_SOURCE_DIR=\"$(CURDIR)\"
 
 # Only test.o gets the define
-test.o: CXXFLAGS += $(TEST_DEFINES)
+test.o: CPPFLAGS += $(TEST_CPPFLAGS)
 
 %.o: %.cpp	simplecpp.h
-	$(CXX) $(CXXFLAGS) -c $<
+	$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $<
 
 testrunner:	test.o	simplecpp.o
 	$(CXX) $(LDFLAGS) simplecpp.o test.o -o testrunner