contrib: lua: fix PIC building of the library

CFLAGS/CPPFLAGS are ignored, all the variables from HOSTVARS and HOSTVARS_PIC are ignored. The values need to be set in the relevant variables.

/usr/bin/ld: /root/build/contrib/x86_64-linux-gnu/lib/liblua5.4.a(liolib.o): warning: relocation against `stdin@@GLIBC_2.2.5' in read-only section `.text'
/usr/bin/ld: /root/build/contrib/x86_64-linux-gnu/lib/liblua5.4.a(lauxlib.o): relocation R_X86_64_PC32 against symbol `stderr@@GLIBC_2.2.5' can not be used when making a shared object; recompile with -fPIC