Fix C++ isnan, isinf, etc for C++-case


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6476 4407c894-4637-0410-b4f5-ada5f102cad1
Make sure template gets declared in C++-mode.  This fix issues related to strayed extern "C" blocks in user-code, when including system-headers


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6477 4407c894-4637-0410-b4f5-ada5f102cad1
Fix wrong result of fpclassify macro.


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6478 4407c894-4637-0410-b4f5-ada5f102cad1
Fix math.h math constant visibility when using -std=c99 -D_GNU_SOURCE

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6479 4407c894-4637-0410-b4f5-ada5f102cad1