Remove extra interpretations to fflags

[mwtoews]

Flags should be passed as expected by the compiler. Currently, pymake's checking/adding '-' to each fflag token is problematic for (e.g.) Intel, which can have flags with spaces like -warn unused. Also, compiler flags are always case-sensitive, so there's no need to do .upper() on these. (Also, these spaces need to be split in cmdlist to work correctly with Popen, which is done here too.)

This is a minor breaking change, in that external tools that use pymake will need to ensure the fflags have dashes, where they would normally be expected by the compiler.

There are many other changes included here (got carried away):

• fflags can either be a str '-flag1 -flag two' or list ['-flag1', '-flag two']
• Correct ifort -double_size to -double-size, and /real_size to /real-size
• Use more descriptive -real-size 64 instead of -r8 (does same thing)
• Decode messages for Python 3
• Simplify flag_available(); document some limitations in docstring
• Use Windows flags as documented (usually /flag:1 rather than -flag 1)