Add a deprecation warning to usages of the compile time "IF" statement.

See https://github.com/cython/cython/issues/4310

Cython/Compiler/Parsing.py

@@ -2303,6 +2303,10 @@ def p_statement(s, ctx, first_statement = 0):
     elif s.sy == 'DEF':
         return p_DEF_statement(s)
     elif s.sy == 'IF':
+        warning(s.position(),
+                "The 'IF' statement is deprecated and will be removed in a future Cython version. "
+                "Consider using runtime conditions or C macros instead. "
+                "See https://github.com/cython/cython/issues/4310", level=1)
         return p_IF_statement(s, ctx)
     elif s.sy == '@':
         if ctx.level not in ('module', 'class', 'c_class', 'function', 'property', 'module_pxd', 'c_class_pxd', 'other'):

tests/run/ct_IF.pyx

+# mode: run
+# tag: warnings
+
 DEF NO = 0
 DEF YES = 1
 
@@ -72,3 +75,12 @@ def control_flow_DEF2():
         DEF B=3
         print('B should be 3.')
     return B
+
+
+_WARNINGS = """
+13:4: The 'IF' statement is deprecated and will be removed in a future Cython version. Consider using runtime conditions or C macros instead. See https://github.com/cython/cython/issues/4310
+27:4: The 'IF' statement is deprecated and will be removed in a future Cython version. Consider using runtime conditions or C macros instead. See https://github.com/cython/cython/issues/4310
+41:4: The 'IF' statement is deprecated and will be removed in a future Cython version. Consider using runtime conditions or C macros instead. See https://github.com/cython/cython/issues/4310
+56:4: The 'IF' statement is deprecated and will be removed in a future Cython version. Consider using runtime conditions or C macros instead. See https://github.com/cython/cython/issues/4310
+71:4: The 'IF' statement is deprecated and will be removed in a future Cython version. Consider using runtime conditions or C macros instead. See https://github.com/cython/cython/issues/4310
+"""