Log Message:
devel/patchelf: Require gcc10 for std::optional

This package was marked to require gcc8 because of std::optional.
Change to 10 to align with the limited versions guidance in
mk/compiler/gcc.mk, to avoid building 8 on a typical system which has
had to build 10 anyway.

Add a comment explaining that the mk/compiler/gcc.mk choice of 7 for
c++17 is buggy because declaring c++17 does not lead to a compiler
that can compile c++17 programs, and that there is no carve out for
std::optional.

Builds on n9/i386 and n10/amd64.