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Related to a shadowed variable bug fix Valdis Kletnieks noticed
that perf does not get built with -Wshadow, which could have
helped us avoid the bug.

So enable -Wshadow and also enable the following warnings on
perf builds, in addition to the already enabled -Wall -Wextra
-std=gnu99 warnings:

 -Wcast-align
 -Wformat=2
 -Wshadow
 -Winit-self
 -Wpacked
 -Wredundant-decls
 -Wstack-protector
 -Wstrict-aliasing=3
 -Wswitch-default
 -Wswitch-enum
 -Wno-system-headers
 -Wundef
 -Wvolatile-register-var
 -Wwrite-strings
 -Wbad-function-cast
 -Wmissing-declarations
 -Wmissing-prototypes
 -Wnested-externs
 -Wold-style-definition
 -Wstrict-prototypes
 -Wdeclaration-after-statement

And change/fix the perf code to build cleanly under GCC 4.3.2.

The list of warnings enablement is rather arbitrary: it's based
on my (quick) reading of the GCC manpages and trying them on
perf.

I categorized the warnings based on individually enabling them
and looking whether they trigger something i...