Include both multibuild and jpeg default CFLAGS

[radarhere]

Resolves python-pillow/Pillow#3441

Here are speed comparisons between the different Pillow versions installed from pip, a local wheel file of 6.0.0 (the only differences between this and pip 6.0.0 should be Freetype, as these wheels were created on TravisCI), and a local wheel with this PR.

Pillow	2.7	3.7
5.0.0	2.01	1.71
5.1.0	4.3	1.59
5.2.0	4.3	4.63
5.3.0	4.3	4.61
5.4.0	4.38	4.56
5.4.1	4.37	4.58
6.0.0	4.36	4.61
6.0.0 Local	4.38	4.64
6.0.0 Local With CFLAGS fix	2.11	2.22

[hugovk]

Thanks!

[marceloboeira]

@radarhere @hugovk awesome!