This avoids defining a version in each translation unit when using
__declspec(dllexport) which causes failures due to multiply defined
symbols with clang-cl:
lld-link: error: duplicate symbol: VP8GetCPUInfo
>>> defined at CMakeFiles\webpdecode.dir\Debug\src\dec\alpha_dec.c.obj
>>> defined at CMakeFiles\webpdsp.dir\Debug\src\dsp\dec_sse41.c.obj
...
Bug: webp:607
Change-Id: I6cd1ee75b3db984aa513263a05516e867a64925d
this synchronizes the code with chrome, where this format allows the
code to pass buildtools/checkdeps/checkdeps.py
Bug: 1141126
Change-Id: I25361b1a43cd95730814302f02aa16af8fdb5fd2