code in src/shared/macro.h only defined MAX/MIN in case
they were not defined previously. however the MAX/MIN
macros implemented in glibc are not of the "safe" kind but defined
as:

define MIN(a,b) (((a)<(b))?(a):(b))
define MAX(a,b) (((a)>(b))?(a):(b))

Avoid nasty side effects by using our own versions instead.

Also fix the warnings derived from this change.

[zj: - modify MAX3 macro to fix warning about _a shadowing _a,
     - do bootchart/svg.c too,
     - remove unused MIN3.]