add arch flag to scons build command

SConstruct

@@ -75,6 +75,12 @@ vars.Add(EnumVariable(
 vars.Add(BoolVariable(
  'profile',
  'build with profiling information', False))
+vars.Add(
+ 'arch',
+ 'target CPU passed to -march in the \'fast\' build, e.g. \'native\' for local '
+ 'builds or \'x86-64-v3\' for a portable AVX2 deployment. Empty means no -march. '
+ 'Defaults to the OBJCRYST_ARCH environment variable if set.',
+ os.environ.get('OBJCRYST_ARCH', ''))
 vars.Update(env)
 
 # Use C++ compiler specified by the 'tool' option.
@@ -184,6 +190,8 @@ else:
  else:
  env.AppendUnique(CCFLAGS=['-fno-strict-aliasing'])
  fast_opts = ['-ffast-math']
+ if env.get('arch'):
+ fast_opts.append('-march=' + env['arch'])
 
  if env['PLATFORM'] == 'darwin':
  # macOS bundle

news/compile_arch.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* Add SCons ``arch`` option to set architecture during compilation. By default, no arch is specified.
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>