Really fix mupdf build.
Really fix mupdf build.

The buildsystem always picks up what's in the env, so if the CFLAGS
in our env has march flags for the wrong arch => kablooey.

Frenzie/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,frankyifei/koreader-base,apletnev/koreader-base,apletnev/koreader-base,Frenzie/koreader-base,koreader/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,Hzj-jie/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,koreader/koreader-base,houqp/koreader-base,Hzj-jie/koreader-base,Hzj-jie/koreader-base,houqp/koreader-base,apletnev/koreader-base,koreader/koreader-base,frankyifei/koreader-base,frankyifei/koreader-base,Hzj-jie/koreader-base,NiLuJe/koreader-base,frankyifei/koreader-base,houqp/koreader-base,apletnev/koreader-base