external toolchain: check BR2_INSTALL_LIBSTDCPP

Verify that the value of BR2_INSTALL_LIBSTDCPP set by the user in the
Buildroot configuration really matches the external toolchain
capabilities by checking that a C++ cross-compiler is available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/external-toolchain/ext-tool.mk

@@ -231,6 +231,15 @@ check_arm_abi = \
 		exit 1 ; \
 	fi ; \
 
+#
+# Check that the external toolchain supports C++
+#
+check_cplusplus = \
+	if ! test -x $(TARGET_CXX) ; then \
+		echo "BR2_INSTALL_LIBSTDCPP is selected but C++ support not available in external toolchain" ; \
+		exit 1 ; \
+	fi ; \
+
 #
 # Check that the cross-compiler given in the configuration exists
 #
@@ -288,6 +297,9 @@ endif
 ifeq ($(BR2_arm),y)
 	$(Q)$(call check_arm_abi)
 endif
+ifeq ($(BR2_INSTALL_LIBSTDCPP),y)
+	$(Q)$(call check_cplusplus)
+endif
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_UCLIBC),y)
 	$(Q)$(call check_uclibc,$(SYSROOT_DIR))
 else