[SCons] Remove Cygwin support

Cygwin support is untested and Cygwin has uncertain usage for Cantera.
Modern replacements such as the Windows Subsystem for Linux v2 (WSL2)
should be used instead.

SConstruct

@@ -200,7 +200,6 @@ config_options = [
            """,
         {
             "cl": "/EHsc",
-            "Cygwin": "-std=gnu++11", # See http://stackoverflow.com/questions/18784112
             "default": "-std=c++11"
         }),
     Option(
@@ -760,7 +759,8 @@ env = Environment(tools=toolchain+["textfile", "subst", "recursiveInstall", "wix
 env["OS"] = platform.system()
 env["OS_BITS"] = int(platform.architecture()[0][:2])
 if "cygwin" in env["OS"].lower():
-    env["OS"] = "Cygwin" # remove Windows version suffix
+    logger.error(f"Error: Operating system {os.name!r} is no longer supported.")
+    sys.exit(1)
 
 if "FRAMEWORKS" not in env:
     env["FRAMEWORKS"] = []
@@ -800,8 +800,6 @@ if env["OS"] == "Darwin":
         config.select("apple-clang")
 
 if "gcc" in env.subst("$CC") or "gnu-cc" in env.subst("$CC"):
-    if env["OS"] == "Cygwin":
-        config.select("Cygwin")
     config.select("gcc")
 
 elif env["CC"] == "cl": # Visual Studio

interfaces/cython/SConscript

@@ -74,9 +74,6 @@ elif localenv['OS'] == 'Windows':
         # See https://github.com/python/cpython/pull/11283
         if py_version_full < parse_version("3.7.3"):
             localenv.Append(CPPDEFINES={"_hypot": "hypot"})
-elif localenv['OS'] == 'Cygwin':
-    # extract 'pythonX.Y' from 'libpythonX.Y.dll.a'
-    localenv.Append(LIBS=pylib[3:-6])
 
 if "numpy_1_7_API" in localenv:
     localenv.Append(CPPDEFINES="NPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION")

site_scons/buildutils.py

@@ -1284,10 +1284,3 @@ def get_pip_install_location(
         print(json.dumps(scheme))
     """)
     return json.loads(get_command_output(python_cmd, "-c", install_script))
-
-
-# Monkey patch for SCons Cygwin bug
-# See https://github.com/SCons/scons/issues/2664
-if "cygwin" in platform.system().lower():
-    import SCons.Node.FS
-    SCons.Node.FS._my_normcase = lambda x: x

test_problems/SConscript

@@ -26,7 +26,7 @@ localenv['ENV']['CANTERA_DATA'] = (Dir('#build/data').abspath + os.pathsep +
                                    Dir('#test/data').abspath)
 
 # Add build/lib in order to find Cantera shared library
-if env['OS'] == 'Windows' or env['OS'] == 'Cygwin':
+if env["OS"] == "Windows":
     localenv.PrependENVPath('PATH', Dir('#build/lib').abspath)
 elif env['OS'] == 'Darwin':
     localenv.PrependENVPath('DYLD_LIBRARY_PATH', Dir('#build/lib').abspath)