meson: make backtrace dependency on execinfo

musl libc for example has no such header.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

build/meson/programs/meson.build

@@ -51,7 +51,8 @@ endif
 
 export_dynamic_on_windows = false
 # explicit backtrace enable/disable for Linux & Darwin
-if not use_backtrace
+execinfo = cc.has_header('execinfo.h', required: use_backtrace)
+if not execinfo.found()
   zstd_c_args += '-DBACKTRACE_ENABLE=0'
 elif use_debug and host_machine_os == os_windows  # MinGW target
   zstd_c_args += '-DBACKTRACE_ENABLE=1'