Added GSL recipe.

recipes/gsl/recipe.py

@@ -0,0 +1,20 @@
+
+import os
+import re
+from SCons.Script import *
+
+def _git_cook(env: Environment, repo: dict) -> dict:
+    checkout_root = repo['checkout_root']
+    return {
+        'CPPPATH': [os.path.join(checkout_root, 'include')]
+    }
+
+
+env.GitRecipe(
+    globals = globals(),
+    repo_name = 'gsl',
+    repo_url = 'https://github.com/microsoft/GSL.git',
+    tag_pattern = re.compile(r'^v([0-9]+)\.([0-9]+)\.([0-9]+)$'),
+    tag_fn = lambda version: f'v{version[0]}.{version[1]}.{version[2]}',
+    cook_fn = _git_cook
+)

recipes/magic_enum/recipe.py

@@ -1,4 +1,5 @@
 
+import os
 import re
 from SCons.Script import *