azure: Fix some places where an unexpected compiler is being picked up

Now we have some information in CI logs about what compiler is actually
being used, fix some places where an unexpected compiler is being picked
up.

Avoid picking up gcc-objc and gfortran from PATH in vs2017 image for VS
test runs.

Use clang for objc/objc++ in MSYS2 clang test runs, rather than picking
up gcc from path.

Also install gfortran for fortran tests on Cygwin.

azure-pipelines.yml

@@ -73,6 +73,7 @@ jobs:
     - script: |
         %CYGWIN_ROOT%\cygwinsetup.exe -qnNdO -R "%CYGWIN_ROOT%" -s "%CYGWIN_MIRROR%" -g -P ^
         cmake,^
+        gcc-fortran,^
         gcc-objc++,^
         gcc-objc,^
         git,^
@@ -154,7 +155,7 @@ jobs:
         set BOOST_ROOT=
         set PATH=%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem
         set PATHEXT=%PATHEXT%;.py
-        if %compiler%==clang ( set CC=clang && set CXX=clang++ )
+        if %compiler%==clang ( set CC=clang && set CXX=clang++ && set OBJC=clang && set OBJCXX=clang++ )
         %MSYS2_ROOT%\usr\bin\bash -lc "MSYSTEM= python3 run_tests.py --backend=ninja"
       env:
         CHERE_INVOKING: yes

ci/azure-steps.yml

@@ -5,8 +5,8 @@ steps:
         exit 0
      }
 
-     # remove MinGW from path, so we don't find gfortran and try to use it
+     # remove Chocolately, MinGW, Strawberry Perl from path, so we don't find gcc/gfortran and try to use it
-     $env:Path = ($env:Path.Split(';') | Where-Object { $_ -notlike '*mingw*' }) -join ';'
+     $env:Path = ($env:Path.Split(';') | Where-Object { $_ -notmatch 'mingw|Strawberry|Chocolatey' }) -join ';'
 
      # download and install prerequisites
      function DownloadFile([String] $Source, [String] $Destination) {