Propagate active compiler to conda-build

Without this, conda-build selects Travis's outdated
C++98 compiler which can't build pybind11.

conda.recipe/meta.yaml

@@ -6,6 +6,9 @@ build:
   number: {{ environ.get('GIT_DESCRIBE_NUMBER', 0) }}
   {% if environ.get('GIT_DESCRIBE_NUMBER', '0') == '0' %}string: py{{ environ.get('PY_VER').replace('.', '') }}_0
   {% else %}string: py{{ environ.get('PY_VER').replace('.', '') }}_{{ environ.get('GIT_BUILD_STR', 'GIT_STUB') }}{% endif %}
+  script_env:
+    - CC
+    - CXX
 
 source:
   git_url: ../