4.1.1: compile time warnings

[kloczek]

Below is summary information about compiler warnings using gcc 11.0.1 and LTO:

$ grep -- -W openmpi.log  | sed 's/.*\[//; s/\]//' | sort | uniq -c | sort -nr
   1631 -Wc-binding-type
    813 -Wunused-result
     43 -Wunused-dummy-argument
     29 -Wlto-type-mismatch
     11 -Wunused-but-set-variable
      8 -Wcpp
      7 -Wunused-variable
      7 -Wunused-function
      6 -Wstringop-overread
      5 -Wstringop-overflow=
      5 -Wdiscarded-qualifiers
      4 -Wincompatible-pointer-types
      2 -Walloc-size-larger-than=
      1 -Wformat-truncation=
      1 -Wformat-extra-args
      1 -Wformat=
      1 -Waddress-of-packed-member

Most important are -Wlto-type-mismatch warmings because even one warming during linking is causing that LTO is not used.
Full stderr output with autogen.sh output when autoconf 2.71 is used is in attachment. Later will try to submit some patch for those autoconf warnings which will be backward compatible with older versions of the autoconf but decision which one version of some declarations in heard files or in .c files needs to be used to resolve those LTO warmings neds to be made bo someone who knowe well OpenMPI code.
openmpi.log

[jsquyres]

LTO was fixed on main in #13193, and will be included in v6.0.0.