Fixed bug 3979 - Failed to link with "multiple definition of `scalbln'" error on both MinGW and MinGW-w64 by CMake

Vitaly Novichkov

Once I ran build of my codecs collection on AppVeyor where my CMake script downloads latest SDL2 from HG repo, failed to link because of math functions conflict:

https://ci.appveyor.com/project/Wohlstand/audiocodecs/build/1.0.44
The revision is b9ff5f8b2303

There are both vanilla MinGW and MinGW-w64 are failed to build.


```
[100%] Linking C shared library libSDL2.dll
c:/mingw/bin/../lib/gcc/mingw32/5.3.0/../../../libmingwex.a(scalbn.o):(.text+0x0): multiple definition of `scalbln'
CMakeFiles\SDL2.dir/objects.a(s_scalbn.c.obj):C:/projects/audiocodecs/build-MinGW-Release-Win32/external/SDL2/src/SDL2HG/src/libm/s_scalbn.c:30: first defined here

src/libm/math_private.h

@@ -40,6 +40,7 @@ typedef unsigned int u_int32_t;
 #define __ieee754_fmod  SDL_uclibc_fmod
 #define __ieee754_log   SDL_uclibc_log
 #define __ieee754_pow   SDL_uclibc_pow
+#define scalbln         SDL_uclibc_scalbln
 #define scalbn          SDL_uclibc_scalbn
 #define sin             SDL_uclibc_sin
 #define __ieee754_sqrt  SDL_uclibc_sqrt