envconfig: read CYTHON from the environment and use it if set

Add support for specifying Cython compiler using the CYTHON environment
variable. If not set, proceed with the names hard coded for Cython.

docs/markdown/Reference-tables.md

@@ -374,6 +374,7 @@ machine](#Environment-variables-per-machine) section for details.
 | Rust          | RUSTC    | RUSTC_LD  | Before 0.54 RUST_LD*                        |
 | Vala          | VALAC    |           | Use CC_LD. Vala transpiles to C             |
 | C#            | CSC      | CSC       | The linker is the compiler                  |
+| Cython        | CYTHON   |           |                                             |
 | nasm          | NASM     |           | Uses the C linker                           |
 
 *The old environment variables are still supported, but are deprecated

mesonbuild/envconfig.py

@@ -88,6 +88,7 @@ ENV_VAR_COMPILER_MAP: T.Mapping[str, str] = {
     'c': 'CC',
     'cpp': 'CXX',
     'cs': 'CSC',
+    'cython': 'CYTHON',
     'd': 'DC',
     'fortran': 'FC',
     'objc': 'OBJC',