[python] Use correct C++ compiler for linking.

CC is the C compiler. CXX is the C++ compiler. Setuptools will
use the C++ compiler for C++ code. Hence it is vital to set that
correctly. If not set the default C++ compiler will be used.

Unfortunately it will use the first string as the CXX compiler and
hence we need to strip ccache. Fortunately it used CC for the
compilation and that works with gcc as well.

python/setup.py.in

@@ -9,6 +9,15 @@ import glob
 import os
 import re
 import subprocess
+
+class custom_build_ext(build_ext):
+    def build_extensions(self):
+        # Make sure that ccache is not in self.compiler.compiler_cxx
+        # as self.compiler.compiler_cxx[0] will be used as
+        # linker command for c++ and that should not be ccache
+        self.compiler.compiler_cxx[0]="@CMAKE_CXX_COMPILER@"
+        build_ext.build_extensions(self)
+
 try:
     from importlib.machinery import EXTENSION_SUFFIXES
     suffix = EXTENSION_SUFFIXES[0]
@@ -19,13 +28,16 @@ setupdir = os.path.dirname(__file__)
 if setupdir != '':
   os.chdir( setupdir )
 
-cc = "@CMAKE_CXX_COMPILER@"
+cc = "@CMAKE_C_COMPILER@"
+cxx = "@CMAKE_CXX_COMPILER@"
 try:
     subprocess.call(['ccache', '--version'])
     os.environ['CC'] = 'ccache {}'.format(cc)
+    os.environ['CXX'] = 'ccache {}'.format(cxx)
     print("Using 'ccache {}' as compiler".format(cc))
 except OSError as e:
     os.environ['CC'] = cc
+    os.environ['CXX'] = cxx
     print('\nNOTE: please install ccache for faster compilation of python bindings.\n')
 
 # This is very hacky but so is the entire setup.py buildsystem.
@@ -108,4 +120,5 @@ setup(
         "Programming Language :: Python :: 3",
         "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
     ],
+    cmdclass={"build_ext": custom_build_ext},
 )