Enable readline on d8 while building a shared lib.

This patch enables readline on d8 except for completion support.
It sould be useful enough for history and line editing.

This is related to V8's issue 1781 (http://code.google.com/p/v8/issues/detail?id=1781), not chromium's.

BUG=1781

Review URL: https://chromiumcodereview.appspot.com/11776017
Patch from Luis Reis <luis.m.reis@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13333 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Only enable MinGW-w64 pow() workaround if needed

The pow() workaround is no longer needed in the latest version of
MinGW-w64.

Contributed by net147@gmail.com

BUGS=
TEST=mjsunit/math-pow,mjsunit/math-sqrt

Review URL: https://chromiumcodereview.appspot.com/11748033
Patch from Jonathan Liu <net147@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13334 ce2b1a6d-e550-0410-aec6-3dcde31c8c00