Versions of GNU make prior to verion 4 do not support
'define =' syntax, but do however support 'define' without
the equals operator. This commit removes the equals operator
to ensure this Makefile is compatible with older versions
of GNU make.
Reported and tested on Windows XP-MinGW with GNU make 3.81
by @ant-222; thank you!
This commit adds `wget` as a fall-back if `curl` fails.
Additionally, it lets the user override the curl and wget
arguments by passing `CURL_FLAGS="args"` and
`WGET_FLAGS="args"` to make.
If both curl and wget fail, it suggesting the user manually
fetch the files or try other curl or wget arguments.
