Fix building on musl libc.

configure.ac

 # Copyright (c) 2006, Google Inc.
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 56 matching lines @@
                  *)
                    AC_MSG_ERROR(bad value ${enableval} for --enable-m32)
                    ;;
                esac],
               [usem32=false])
 
 AC_HEADER_STDC
 AC_SYS_LARGEFILE
 m4_include(m4/ax_pthread.m4)
 AX_PTHREAD
-AC_CHECK_HEADERS([a.out.h])
+AC_CHECK_HEADERS([a.out.h linux/a.out.h])
 
 m4_include(m4/ax_cxx_compile_stdcxx.m4)
 AX_CXX_COMPILE_STDCXX(11, noext, mandatory)
 
 # Only build Linux client libs when compiling for Linux
 case $host in
   *-*-linux* | *-android* )
     LINUX_HOST=true
     ;;
 esac
@@ skipping 107 matching lines @@
               [selftest=false])
 AM_CONDITIONAL(SELFTEST, test x$selftest = xtrue)
 
 AC_CONFIG_FILES(m4_flatten([
   breakpad.pc
   breakpad-client.pc
   Makefile
 ]))
 
 AC_OUTPUT