Upstream's cmake magic checks for execinfo.h, but there is no test to
check if the backtrace(3) function family requires extra linker flags.
Replace the header check by testing if backtrace(3) exists, and if it
does, add the required linker flag.
This fixes telephony/linphone/belr build on ld.bfd archs, at least.
Tested on macppc and amd64.
« ok if it works for you » landry@ (maintainer)
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