Add integration test coverage for make flag passing

This requires making the make_simple Makefile more realistic by

* using CC instead of setting it and
* using CXXFLAGS instead of CXX_FLAGS and not overwriting its contents.

examples/make_simple/BUILD.bazel

@@ -3,10 +3,9 @@ load("@rules_foreign_cc//foreign_cc:defs.bzl", "make")
 
 make(
     name = "make_lib",
-    build_data = ["//make_simple/code:clang_wrapper.sh"],
-    env = {
-        "CLANG_WRAPPER": "$(execpath //make_simple/code:clang_wrapper.sh)",
-    },
+    copts = [
+        "-DREQUIRED_DEFINE",
+    ],
     lib_source = "//make_simple/code:srcs",
     out_static_libs = ["liba.a"],
 )
@@ -16,6 +15,9 @@ cc_test(
     srcs = [
         "test_libb.cpp",
     ],
+    copts = [
+        "-std=c++11",
+    ],
     tags = ["windows"],
     deps = [
         ":make_lib",

examples/make_simple/code/Makefile

@@ -1,19 +1,18 @@
 BUILD_DIR=build-out
 
 UNAME:=$(shell uname)
-CXX_FLAGS :=
 
 ifneq (,$(findstring NT, $(UNAME)))
   # If Windows
-  CXX_FLAGS := /MD
+  CXXFLAGS := $(CXXFLAGS) /MD
 else
-  CXX_FLAGS := -fPIC
+  CXXFLAGS := $(CXXFLAGS) -fPIC
 endif
 
 default all $(BUILD_DIR)/lib/liba.a: liba.cpp liba.h
 	rm -rf $(BUILD_DIR)/lib
 	mkdir -p $(BUILD_DIR)/lib
-	$(CLANG_WRAPPER) $(CXX_FLAGS) -o $(BUILD_DIR)/lib/liba.o -c liba.cpp
+	$(CXX) $(CXXFLAGS) -o $(BUILD_DIR)/lib/liba.o -c liba.cpp
 	ar rcs $(BUILD_DIR)/lib/liba.a $(BUILD_DIR)/lib/liba.o
 
 install: $(BUILD_DIR)/lib/liba.a

examples/make_simple/code/liba.cpp

@@ -1,5 +1,19 @@
 #include "liba.h"
 
+#include <math.h>
+
+#ifndef REQUIRED_DEFINE
+// '-DREQUIRED_DEFINE' is set via the copts attribute of the make rule.
+#error "REQUIRED_DEFINE is not defined"
+#endif
+
 std::string hello_liba(void) {
 	return "Hello from LIBA!";
-}
+}
+
+double hello_math(double a) {
+	// On Unix, this call requires linking to libm.so. The Bazel toolchain adds
+	// the required `-lm` linkopt automatically and rules_foreign_cc forwards
+	// this option to make invocation via the CXXFLAGS variable.
+	return acos(a);
+}

examples/make_simple/code/liba.h

@@ -5,5 +5,6 @@
 #include <string>
 
 std::string hello_liba(void);
+double hello_math(double);
 
-#endif
+#endif

examples/make_simple/test_libb.cpp

@@ -10,5 +10,9 @@ int main(int argc, char* argv[])
   if (result != "Hello from LIBA!") {
     throw std::runtime_error("Wrong result: " + result);
   }
+  double math_result = hello_math(0.5);
+  if (math_result < 1.047 || math_result > 1.048) {
+    throw std::runtime_error("Wrong math_result: " + std::to_string(math_result));
+  }
   std::cout << "Everything's fine!";
 }