Add span_FEATURE_MAKE_SPAN

[martinmoene]

Add configuration flag span_FEATURE_MAKE_SPAN to unconditionally control presence of make_span()

Plan:

• Add span_FEATURE_MAKE_SPAN with priority above span_FEATURE_MAKE_SPAN_TO_STD
• Add span_FEATURE_MAKE_SPAN to test/span-main.t.cpp
• Document span_FEATURE_MAKE_SPAN, mentioning it precedes over span_FEATURE_MAKE_SPAN_TO_STD
• In documentation of span_FEATURE_MAKE_SPAN_TO_STD, mention span_FEATURE_MAKE_SPAN takes precedence
• Mention macro in table
• Mention span_FEATURE_MAKE_SPAN in test/CMakeLists.txt
• Mention span_FEATURE_MAKE_SPAN in test/t.bat, tg.bat, tc.bat

Originates from issue #32 .

Envisioned implementation:

#ifndef  span_FEATURE_MAKE_SPAN
#ifdef   span_FEATURE_MAKE_SPAN_TO_STD
# define span_FEATURE_MAKE_SPAN  span_IN_STD( span_FEATURE_MAKE_SPAN_TO_STD )
#else
# define span_FEATURE_MAKE_SPAN  0
#endif
#endif

[Flamefire]

Please do the same for all other *_TO_STD macros. Especially as e.g. the subspan function rely on this.