mimxrt/Makefile: Add CXXFLAGS, and libstdc++ to LDFLAGS.

This allows user C modules to be built into the mimxrt port.  The change is
copied from `samd/Makefile`.

Fixes issue micropython#18292.

Signed-off-by: Dryw Wade <dryw.wade@sparkfun.com>

Author: sfe-SparkFro

ports/mimxrt/Makefile

@@ -437,6 +437,15 @@ CFLAGS += \
 	-Wfloat-conversion \
 	-Wno-error=unused-parameter
 
+# Flags for optional C++ source code
+CXXFLAGS += $(filter-out -std=c99,$(CFLAGS))
+
+# TODO make this common -- shouldn't be using these "private" vars from py.mk
+ifneq ($(SRC_CXX)$(SRC_USERMOD_CXX)$(SRC_USERMOD_LIB_CXX),)
+LIBSTDCPP_FILE_NAME = "$(shell $(CXX) $(CXXFLAGS) -print-file-name=libstdc++.a)"
+LDFLAGS += -L"$(shell dirname $(LIBSTDCPP_FILE_NAME))"
+endif
+
 # Configure respective board flash type
 # Add hal/flexspi_nor_flash.h or hal/flexspi_hyper_flash.h respectively
 CFLAGS += -DBOARD_FLASH_OPS_HEADER_H=\"hal/flexspi_$(subst qspi_,,$(FLEXSPI_FLASH_TYPE)).h\"