libstdc++/7680
	* include/c_std/std_cmath.h (__gnu_cx::__c99_binding): New namespace.
	Populate it with multiple legal ways to obtain the C99 float
	transcendentals.  Use them instead of direct global reference.
	(C99 FP capture): Guard usage with _GLIBCPP_USE_C99_FP_MACROS_DYNAMIC.
	* docs/html/17_intro/porting.texi
	(_GLIBCPP_USE_C99_FLOAT_TRANSCENDENTALS_CHECK): New macro.
	(_GLIBCPP_USE_C99_FLOAT_TRANSCENDENTALS_DYNAMIC): New macro.
	(_GLIBCPP_USE_C99_FP_MACROS_DYNAMIC): New macro.
	* config/os/bsd/freebsd/os_defines.h
	(_GLIBCPP_USE_C99_FLOAT_TRANSCENDENTALS_CHECK): New macro.
	(_GLIBCPP_USE_C99_FLOAT_TRANSCENDENTALS_DYNAMIC): New macro.
	* testsuite/26_numerics/c_math_dynamic.cc: New file.

From-SVN: r65624