[tools] more pbc_to_exe fixup for unistd.h with --optimize

Only _WIN32 has no unistd.h. Should close #1117
cygwin: check for wrong gcc-4, --disable-shared

replace wrong gcc-4/g++-4 with gcc/g++. GH #1116
set proper libparrot_ldflags for --disable-shared
  (the static lib is in blib/lib)
fix wrong $disable_shared logic in defaults