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
