Include both multibuild and libwebp default CFLAGS

[radarhere]

Resolves python-pillow/Pillow#4040. libwebp equivalent of #117

Takes the new CFLAGs from https://github.com/webmproject/libwebp/blob/0e48d889eb90363c6ebf8ea276bae4d892f9d3c0/makefile.unix#L112

Testing with the following code -

from PIL import Image
import time
im = Image.open('hopper.jpg')
start = time.time()
for i in range(0, 1000):
    im.save('out.webp')
print(time.time() - start)

Speed	macOS	Linux
Timer script	19.58s	22.19s
Fix	5.34s	3.54s

[hugovk]

Looks good!

What are -O3 -DNDEBUG? Do you have a link to some docs?

[hugovk]

We could mention this in the release notes as well.

[nulano]

-O3 enables most GCC optimizations (docs).
-DNDEBUG defines NDEBUG symbol which is commonly used to mean Not Debug / Release build (e.g. in C++ assert funcion)