propagate compiler variables to make PythonInstall

wrappers/python/pysetup.cmake.in

 set(ENV{OPENMM_INCLUDE_PATH} "@WRAPPER_INCLUDE_DIRS@")
+set(ENV{CC} "@CMAKE_C_COMPILER@")
+set(ENV{CXX} "@CMAKE_CXX_COMPILER@")
 file(TO_NATIVE_PATH "@CMAKE_INSTALL_PREFIX@/lib" OPENMM_LIB_PATH)
 set(ENV{OPENMM_LIB_PATH} "${OPENMM_LIB_PATH}")
 message("OPENMM_LIB_PATH = " $ENV{OPENMM_LIB_PATH})
 message("OPENMM_INCLUDE_PATH = " $ENV{OPENMM_INCLUDE_PATH})
+message("CC = " $ENV{CC})
+message("CXX = " $ENV{CXX})
+
 execute_process(
     COMMAND "@PYTHON_EXECUTABLE@" setup.py @PYTHON_SETUP_COMMAND@
     WORKING_DIRECTORY "@PYTHON_PACKAGE_DIR@"