conda-recipe: workaround newer libpng issue

conda-fore currently does not copy libpng16.lib as png.lib, which the older
package from the defaults channel did. Lets do that for the moment and enable
the conda build again.

png.lib during the conda build (as opposed to the static one in the tests and
wheel builds, which contains all object code) is used as a kind of header file,
the real linked dependency is then on `libpng16.dll` and therefore we need a
libpng dependency in the conda binary package.

Author: jankatins

appveyor.yml

@@ -71,7 +71,7 @@ install:
   # VS tools instead of the also installed Py27 VS compiler (which wouldn't need this workarounds...)
   - cmd: copy "C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\bin\vcvars64.bat" "C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\bin\amd64\vcvarsamd64.bat"
 
-  # For building, use a new environment which only include sthe requirements for mpl
+  # For building, use a new environment which only includes the requirements for mpl
   # same things as the requirements in ci/conda_recipe/meta.yaml
   - cmd: conda create -y -q -n test-environment python=%PYTHON_VERSION% pip setuptools numpy python-dateutil freetype msinttypes tk pyparsing pytz tornado libpng zlib pyqt cycler nose mock msvc_runtime
   - activate test-environment
@@ -96,13 +96,13 @@ test_script:
   - python -c "import matplotlib as m; m.use('tkagg'); import matplotlib.pyplot as plt; print(plt.get_backend())"
   # tests
   - python tests.py
-  # remove to get around libpng issue?
+  # Generate a html for visual tests
   - python visual_tests.py
 
 after_test:
   # After the tests were a success, build packages (wheels and conda)
 
-  # Build the wheel
+  # Build the wheel with the static libs
   # Hide the output, the copied files really clutter the build log...
   - cmd: '%CMD_IN_ENV% python setup.py bdist_wheel > NUL:'
 
@@ -116,7 +116,7 @@ after_test:
   - cmd: path
   - cmd: where python
   - cmd: '%CMD_IN_ENV% conda config --get channels'
-  # - cmd: '%CMD_IN_ENV% conda build .\ci\conda_recipe'
+  - cmd: '%CMD_IN_ENV% conda build .\ci\conda_recipe'
 
   # Move the conda package into the dist directory, to register it
   # as an "artifact" for Appveyor.

ci/conda_recipe/bld.bat

@@ -8,5 +8,9 @@ ECHO tests = False >> setup.cfg
 ECHO sample_data = False >> setup.cfg
 ECHO toolkits_tests = False >> setup.cfg
 
+@rem workaround for https://github.com/matplotlib/matplotlib/issues/6460
+@rem see also https://github.com/conda-forge/libpng-feedstock/pull/4
+copy /y %LIBRARY_LIB%\libpng16.lib %LIBRARY_LIB%\png.lib
+
 %PYTHON% setup.py install
 if errorlevel 1 exit 1