build: test compiling all public headers with C++ compiler

While libnl3 is a C library (and itself can only be built using a C
compiler), the public headers are supposed to also work with C++.

Add a test for that.

Unfortunately, this test does not cover whether all symbols are
correctly marked as "extern "C"" for linkage.

Makefile.am

@@ -1160,6 +1160,31 @@ check_local += check-local-build-headers
 
 ###############################################################################
 
+if HAS_CXX
+%.build-headers-test-cxx.c: %
+	mkdir -p "$(dir $@)"
+	printf "#include <cstdio>\n#include <$$(echo "$<" | sed 's|.*\<include/netlink/|netlink/|')>\nint main(void) { return 0; }" > $@
+
+%.build-headers-test-cxx.o: %.build-headers-test-cxx.c
+	$(CXX) \
+		-Wall \
+		-Werror \
+		-I$(srcdir)/include \
+		-I$(builddir)/include \
+		-D_NL_NO_WARN_DEPRECATED_HEADER \
+		-c \
+		-o $@ $<
+
+BUILD_HEADERS_OBJS_CXX = $(patsubst %,%.build-headers-test-cxx.o,$(public_headers))
+check-local-build-headers-cxx: $(BUILD_HEADERS_OBJS_CXX)
+else
+check-local-build-headers-cxx:
+endif
+
+check_local += check-local-build-headers-cxx
+
+###############################################################################
+
 check-local: $(check_local)
 
 .PHONY: $(check_local)

configure.ac

@@ -63,6 +63,7 @@ AC_SUBST(LT_AGE)
 
 AC_PROG_CC
 AM_PROG_CC_C_O
+AC_PROG_CXX
 AC_PROG_INSTALL
 LT_INIT
 AC_PROG_MKDIR_P
@@ -72,6 +73,15 @@ AC_CHECK_PROGS(YACC, 'bison -y')
 AC_C_CONST
 AC_C_INLINE
 
+AC_LANG_PUSH([C++])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+  [[#ifndef __cplusplus
+    #error "broken C++"
+    #endif]])],,
+  [CXX=;])
+AC_LANG_POP([C++])
+AM_CONDITIONAL([HAS_CXX], [test "x$CXX" != x])
+
 PKG_CHECK_MODULES([CHECK], [check >= 0.9.0],
 	[has_check="yes"],
 	[AC_MSG_WARN([*** Disabling building of unit tests])
@@ -155,6 +165,11 @@ fi
 
 AC_OUTPUT
 
+CXX_MSG="none"
+if test "x$CXX" != x ; then
+	CXX_MSG="[$CXX]"
+fi
+
 echo
 echo "libnl $LIBNL_VERSION configuration${LIBNL_GIT_SHA:+ (git:$LIBNL_GIT_SHA)}:"
 echo "    --enable-pthreads=$enable_pthreads"
@@ -164,4 +179,5 @@ echo "    --enable-debug=$enable_debug"
 echo "    --enable-cli=$enable_cli"
 echo
 echo "    check: $has_check"
+echo "    CXX: $CXX_MSG (only used for tests)"
 echo