py/py.mk: Support C++ code for user C modules.

Support C++ code in .cpp files by providing CXX counterparts of the
_USERMOD_ flags we have for C already.  This merely enables the Makefile of
user C modules to use variables specific to C++ compilation, it is still up
to each port's main Makefile to also include these in the build.

docs/develop/cmodules.rst

@@ -8,7 +8,8 @@ limitations with the Python environment, often due to an inability to access
 certain hardware resources or Python speed limitations.
 
 If your limitations can't be resolved with suggestions in :ref:`speed_python`,
-writing some or all of your module in C is a viable option.
+writing some or all of your module in C (and/or C++ if implemented for your port)
+is a viable option.
 
 If your module is designed to access or work with commonly available
 hardware or libraries please consider implementing it inside the MicroPython
@@ -29,7 +30,7 @@ Structure of an external C module
 
 A MicroPython user C module is a directory with the following files:
 
-* ``*.c`` and/or ``*.h`` source code files for your module.
+* ``*.c`` / ``*.cpp`` / ``*.h`` source code files for your module.
 
   These will typically include the low level functionality being implemented and
   the MicroPython binding functions to expose the functions and module(s).
@@ -44,12 +45,12 @@ A MicroPython user C module is a directory with the following files:
   in your ``micropython.mk`` to a local make variable,
   eg ``EXAMPLE_MOD_DIR := $(USERMOD_DIR)``
 
-  Your ``micropython.mk`` must add your modules C files relative to your
+  Your ``micropython.mk`` must add your modules source files relative to your
   expanded copy of ``$(USERMOD_DIR)`` to ``SRC_USERMOD``, eg
   ``SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c``
 
   If you have custom ``CFLAGS`` settings or include folders to define, these
-  should be added to ``CFLAGS_USERMOD``.
+  should be added to ``CFLAGS_USERMOD``, or ``CXXFLAGS_USERMOD``.
 
   See below for full usage example.
 

py/py.mk

@@ -33,7 +33,9 @@ ifneq ($(USER_C_MODULES),)
 # pre-define USERMOD variables as expanded so that variables are immediate
 # expanded as they're added to them
 SRC_USERMOD :=
+SRC_USERMOD_CXX :=
 CFLAGS_USERMOD :=
+CXXFLAGS_USERMOD :=
 LDFLAGS_USERMOD :=
 $(foreach module, $(wildcard $(USER_C_MODULES)/*/micropython.mk), \
     $(eval USERMOD_DIR = $(patsubst %/,%,$(dir $(module))))\
@@ -42,7 +44,9 @@ $(foreach module, $(wildcard $(USER_C_MODULES)/*/micropython.mk), \
 )
 
 SRC_MOD += $(patsubst $(USER_C_MODULES)/%.c,%.c,$(SRC_USERMOD))
+SRC_MOD_CXX += $(patsubst $(USER_C_MODULES)/%.cpp,%.cpp,$(SRC_USERMOD_CXX))
 CFLAGS_MOD += $(CFLAGS_USERMOD)
+CXXFLAGS_MOD += $(CXXFLAGS_USERMOD)
 LDFLAGS_MOD += $(LDFLAGS_USERMOD)
 endif