fix: change cygpath to mixed mode

[jsun-splunk]

'autoconf' applications often has a ./configure stage that checks the $srcdir (source directory). This is represented in foreign_cc by a combination of $EXT_BUILD_ROOT and a relative path to the source. This check fails on windows if $EXT_BUILD_ROOT contains '\' since it is an escape char under unix. Therefore, we need to generate $EXT_BUILD_ROOT with '/' instead.

Generating $EXT_BUILD_ROOT with '/' present a different problem as now the resulting PATH variable becomes the format "/C:/path/to/a:/C:/path/to/b". Due to ':' being used as both window drive leter assignment and linux path delimiter, MSYS2 gets very confused and doesn't resolve this correctly. Therefore, we need this to become the format "/C/path/to/a:/C/path/to/b".

$EXT_BUILD_ROOT is sometimes passed through the 'env' attribute by the user. We need to handle this scenario as well.