Update appveyor.yml

appveyor.yml

@@ -43,6 +43,7 @@ install:
 
   - if [%WITH_FORTRAN%]==[no] conda install --yes --quiet ninja
   - if [%WITH_FORTRAN%]==[yes] conda install --yes --quiet -c isuruf kitware-ninja
+  - if [%WITH_FORTRAN%]==[yes] conda install --yes --quiet flang
 
   - if [%COMPILER%]==[clang-cl] call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" x64
   - if [%COMPILER%]==[clang-cl] set "LIB=%CONDA_INSTALL_LOCN%\Library\lib;%LIB%"