eu-stacktrace [2/5]: configure.ac: initial version (x86/sysprof only)

eu-stacktrace [2/5]: configure.ac: initial version (x86/sysprof only)

Due to the x86-specific code in the initial version the configury has
significant restrictions. If --enable-stacktrace is not explicitly
provided, then eu-stacktrace will be disabled by default.

The way we test for x86 is a bit unusual. What we actually care about
is that the register file provided by perf_events on the system is an
x86 register file; this is done by checking that <asm/perf_regs.h> is
Linux kernel arch/x86/include/uapi/asm/perf_regs.h.

Once eu-stacktrace is properly portable across architectures,
these grody checks can be simplified. Enablement of the feature
by default depends on a released Sysprof version we can point to
for the patches.

* configure.ac: Add configure checks and conditionals for stacktrace tool.
* src/Makefile.am: Add stacktrace tool conditional on ENABLE_STACKTRACE.

Signed-off-by: Serhei Makarov <serhei@serhei.io>