meson: Search for execinfo.h

Rather than checking __GLIBC__/__UCLIBC__ macros as a proxy for
execinfo.h presence, just check directly. This allows the build to work
on musl.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

meson.build

@@ -1036,7 +1036,7 @@ elif cc.has_header_symbol('sys/mkdev.h', 'major')
   pre_args += '-DMAJOR_IN_MKDEV'
 endif
 
-foreach h : ['xlocale.h', 'sys/sysctl.h', 'linux/futex.h', 'endian.h', 'dlfcn.h']
+foreach h : ['xlocale.h', 'sys/sysctl.h', 'linux/futex.h', 'endian.h', 'dlfcn.h', 'execinfo.h']
   if cc.compiles('#include <@0@>'.format(h), name : '@0@'.format(h))
     pre_args += '-DHAVE_@0@'.format(h.to_upper().underscorify())
   endif