Adds warning to prepare_ssl when nasm is not available.

Force clean of externals on buildbots.

PCbuild/prepare_ssl.py

@@ -174,6 +174,9 @@ def main():
     if not find_all_on_path('nmake.exe'):
         print('Could not find nmake.exe, try running env.bat')
         sys.exit(1)
+    if not find_all_on_path('nasm.exe'):
+        print('Could not find nasm.exe, please add to PATH')
+        sys.exit(1)
     sys.stdout.flush()
 
     # Put our working Perl at the front of our path

Tools/buildbot/build.bat

@@ -7,7 +7,7 @@ call "%~dp0clean.bat" %*
 @rem a new version of an external library, especially Tcl/Tk):
 @rem 1) uncomment the following line:
 
-@rem    call "%~dp0..\..\PCbuild\get_externals.bat" --clean-only
+call "%~dp0..\..\PCbuild\get_externals.bat" --clean-only
 
 @rem 2) commit and push
 @rem 3) wait for all Windows bots to start a build with that changeset