scons: Print a deprecation warning about using scons on not windows

At this point meson should be able to handle all of the non-windows
platforms just fine; we'd like to be able to stop maintaining scons for
those platforms sooner than later.

Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/common.py b/common.py
index 51dba9c..b7762cb 100644
--- a/common.py
+++ b/common.py
@@ -112,6 +112,7 @@
     opts.Add(BoolOption('asan', 'enable Address Sanitizer', 'no'))
     opts.Add('toolchain', 'compiler toolchain', default_toolchain)
     opts.Add(BoolOption('llvm', 'use LLVM', default_llvm))
+    opts.Add(BoolOption('force_scons', 'Force enable scons on deprecated platforms', 'false'))
     opts.Add(BoolOption('openmp', 'EXPERIMENTAL: compile with openmp (swrast)',
                         'no'))
     opts.Add(BoolOption('debug', 'DEPRECATED: debug build', 'yes'))