configure: modernize testing of strings
print SIMD enablements at the end of configure
rearranged SIMD-enabling tests; always try to enable SSE2
completely remove all the automatic disabling of SIMD-instructions
simplify check for supported SIMD instructions
have "--without-ALL" set "--with-DeckLink" and "--with-NDI"
have "--with-pipewire" override the whacko "--with-libpipewire_0_3"
Only install help-files for [gem*window] if the corresponding object is built