[Bug libstdc++/58393] Please relax feature check for std::to_string and std::sto* for uClibc

["nicolas.cavallari at lri dot fr" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58393

nicolas.cavallari at lri dot fr changed:

           What    |Removed                     |Added
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  Attachment #30800|0                           |1
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--- Comment #4 from nicolas.cavallari at lri dot fr ---
Created attachment 33060
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33060&action=edit
7 patches for being more specific with feature checks, in mbox format.

Following Paolo's comment, i made some other patches a while ago, using more
specific feature checks everywhere.  The only part where _GLIBCXX_USE_C99 is
still needed is when determining whether the time functions allows one or two
leap seconds.

However, my attemps to run the testsuite were always met with frustration and
undeterministic results, so i delayed it and kept working around the problem in
my programs.

I attach my untested patches here. Maybe i'll fight with the testsuite again,
maybe not.