Change mechanism to skip  VC < v14
Per [suggestion](https://github.com/conda-forge/staged-recipes/pull/6587#pullrequestreview-152389409) of
@@ -15,6 +15,7 @@ source:
build:
number: 0
+ skip: True # [win and vc<14]
requirements:
build:
@@ -26,7 +27,6 @@ requirements:
- zlib
- jpeg
- libpng
- - python # [win] to try to force vc >= 14
run:
- zlib
