Gitian build fix: c_str() -> string()

This prevents:

mastercore.cpp: In function ‘void prune_state_files(const CBlockIndex*)’:
mastercore.cpp:4060:94: error: conditional expression between distinct pointer types ‘const char*’ and ‘const value_type* {aka const wchar_t*}’ lacks a cast [-fpermissive]
mastercore.cpp:4060:94: error: invalid conversion from ‘const void*’ to ‘const char*’ [-fpermissive]
Windows build: don't throw log path error
Use boost path for temp log files

On Ubuntu 14.04 LTS this mirrors /tmp/mastercore.log, /tmp/dead.log
See: https://github.com/mastercoin-MSC/mastercore/blob/17d7cc46ba848b45f38c03a9de55202738846bfa/src/util.cpp#L1359
Merge pull request #71 from dexX7/mcore-fix-win-build

Fix related to Gitian building in Windows