libstdc++: Stop using _GLIBCXX_USE_C99_COMPLEX_TR1 in <complex>

The _GLIBCXX_USE_C99_COMPLEX_TR1 macro (and the comments about it in
acinclude.m4 and config.h) are misleading when it is also used for
<complex>, not only <tr1/complex>. It is also wrong, because the
configure checks for TR1 use -std=c++98 and a target might define cacos
etc. for C++11 but not for C++98.

Add a separate configure check for the inverse trigonometric functions
that are covered by _GLIBCXX_USE_C99_COMPLEX_TR1, but using -std=c++11
for the checks. Use the result of that separate check in <complex>.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_USE_C99): Check for complex inverse trig
	functions in C++11 mode and define _GLIBCXX_USE_C99_COMPLEX_ARC.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* doc/doxygen/user.cfg.in (PREDEFINED): Add new macro.
	* include/std/complex: Check _GLIBCXX_USE_C99_COMPLEX_ARC
	instead of _GLIBCXX_USE_C99_COMPLEX_TR1.