cygwinccompiler: Get the compilers from sysconfig

On CLANG64 environment we use should use clang
instead of gcc. This patch gets the compiler from
sysconfig and which would be set when python
was built.

Without this patch, the build fails when it's
trying to check if the compiler is cygwin's one.
`customize_compiler` function handles updates
to the compiler from `CC` and `CXX` environment.

distutils/cygwinccompiler.py

@@ -23,6 +23,7 @@
     DistutilsPlatformError,
 )
 from .file_util import write_file
+from .sysconfig import get_config_vars
 from .unixccompiler import UnixCCompiler
 from .version import LooseVersion, suppress_known_deprecation
 
@@ -95,8 +96,11 @@ def __init__(self, verbose=False, dry_run=False, force=False):
                 "Compiling may fail because of undefined preprocessor macros."
             )
 
-        self.cc = os.environ.get('CC', 'gcc')
-        self.cxx = os.environ.get('CXX', 'g++')
+        self.cc, self.cxx = get_config_vars('CC', 'CXX')
+        if self.cc is None:
+            self.cc = 'gcc'
+        if self.cxx is None:
+            self.cxx = 'g++'
 
         self.linker_dll = self.cc
         shared_option = "-shared"