[tools] fixup pbc_to_exe fix, missing unistd.h with --optimize

PARROT_HAS_HEADER_UNISTD is only defined after loading parrot/api.h
Fixes #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