ar response files under MinGW contain paths with backslashes, which fails

[pdimov]

As reported in boostorg/url#779, the gcc.archive action creates response files in which the paths use backslashes as the separator, which the MinGW ar misinterprets as escapes.

The fix is to change the :E part of this line

b2/src/tools/gcc.jam

Line 1094 in 5dca296

"$(.AR)" $(AROPTIONS) $(.ARFLAGS) "$(<)" @($(<[-1]:T).rsp:O=FC:<=@":>=":E="$(>)")

from :E="$(>)" to :E="$(>:T)".

(I see that this has been reported before as #342.)