mingw tool not using the same search paths between exists and generate methods

[jcbrill]

The current implementation of the mingw tool augments the mingw search paths for version specific mingw-w64 folders in the generate function but fails to use the same search paths in the exists function.

mingw-w64 folders installed as C:\mingw-w64\<SPECIFICVERSION>\mingw64 are detected in the generate function. Therefore, the following environment works as expected:

env = Environment(tools=['mingw'])

However, if there are no versions of msvc installed, the following fails due to attempting to build with the msvc tools:

env = Environment()

Because the mingw-w64 specific version folders are not considered in the exists function, the mingw tool is filtered from the win32 compiler candidate list. When the only compiler installed is mingw, this causes the installed list to be empty in which case the default tool is msvc. This causes the default environment to fail without msvc installed.

Additional Information:

• SCons master branch
• Python 3.6.8 64-bit (embedded and winpython)
• Neither embedded or winpython requires installation
• Windows 10 VMWare Virtual Machine
• Mingw-w64 installed to C:\mingw-w64\mingw-1110-v9-r0-64-win32-sjlj\mingw64
• Visual Studio not installed.