Add test for C++ compilation and includes.

Makefile

@@ -67,6 +67,7 @@ clean :
 	$(Q)$(MAKE) clean -C src
 	$(Q)$(MAKE) clean -C test/unit
 	$(Q)$(MAKE) clean -C test/functional/src
+	$(Q)$(MAKE) clean -C test/cpp
 	$(Q)$(MAKE) clean -C examples
 	$(Q)rm -rf docs/html
 	$(Q)rm -rf test/functional/__pycache__
@@ -78,7 +79,7 @@ clean :
 	$(Q)$(MAKE) clean -C third_party/dtoa
 
 # Run the unit tests.
-test : all test_unit test_functional
+test : all test_cpp test_unit test_functional
 
 # Test for C++ const correctness on APIs.
 test_const : all
@@ -96,6 +97,10 @@ test_unit : all
 	$(Q)$(MAKE) -C src test_lib
 	$(Q)$(MAKE) -C test/unit test
 
+# Test C++ compilation.
+test_cpp : all
+	$(Q)$(MAKE) -C test/cpp
+
 # Test Cmake. This test should really be done with Cmake in the cmake dir but
 # this is a workaround for now.
 test_cmake :

test/cpp/Makefile

@@ -0,0 +1,37 @@
+###############################################################################
+#
+# Makefile for libxlsxwriter testcase.
+#
+# Copyright 2014-2021, John McNamara, jmcnamara@cpan.org
+#
+
+# Keep the output quiet by default.
+Q=@
+ifdef V
+Q=
+endif
+
+# Directory variables.
+INC_DIR = ../../include
+
+# Flags passed to the C++ compiler.
+CXXFLAGS += -g -Wall -Wextra
+
+# Source files to compile.
+SRCS = $(wildcard *.cpp)
+EXES = $(patsubst %.cpp,%,$(SRCS))
+
+LIBXLSXWRITER = ../../src/libxlsxwriter.a
+LIBS = $(LIBXLSXWRITER) -lz
+
+all : $(LIBXLSXWRITER) $(EXES)
+
+$(LIBXLSXWRITER):
+	$(Q)$(MAKE) -C ../.. all
+
+clean :
+	$(Q)rm -f $(EXES)
+
+# Executable targets.
+%: %.cpp $(LIBXLSXWRITER)
+	$(Q)$(CXX) -I$(INC_DIR) $(CXXFLAGS) $< -o $@ $(LIBS)

test/cpp/test_compilation.cpp

@@ -0,0 +1,22 @@
+/*
+ * Simple C++ program to test compilation in includes with libxlsxwriter.
+ *
+ * Copyright 2014-2021, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = workbook_new("hello_world.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+
+    worksheet_write_string(worksheet, 0, 0, "Hello", NULL);
+    worksheet_write_number(worksheet, 1, 0, 123, NULL);
+
+    workbook_close(workbook);
+
+    return 0;
+}