linux_core_dumper: add a fallback definition for NT_SIGINFO

When building on an old system with outdated headers, this define
might not be available.  Add a fallback to our existing elf header.

Bug: google-breakpad:790
Change-Id: I4dfe7a5cebd414cca3582a1a9cfc983503d5a779
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1507073
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/client/linux/minidump_writer/linux_core_dumper.cc

diff --git a/src/client/linux/minidump_writer/linux_core_dumper.cc b/src/client/linux/minidump_writer/linux_core_dumper.cc
index 9a2f4d6..3bb2ee7 100644
--- a/src/client/linux/minidump_writer/linux_core_dumper.cc
+++ b/src/client/linux/minidump_writer/linux_core_dumper.cc
@@ -43,6 +43,7 @@
 #include <asm/reg.h>
 #endif
 
+#include "common/linux/elf_gnu_compat.h"
 #include "common/linux/linux_libc_support.h"
 
 namespace google_breakpad {

src/common/linux/elf_gnu_compat.h

diff --git a/src/common/linux/elf_gnu_compat.h b/src/common/linux/elf_gnu_compat.h
index f870cbc..0a3dfed 100644
--- a/src/common/linux/elf_gnu_compat.h
+++ b/src/common/linux/elf_gnu_compat.h
@@ -43,4 +43,9 @@
 #define NT_GNU_BUILD_ID 3
 #endif
 
+// Newer Linux systems offer this.
+#ifndef NT_SIGINFO
+#define NT_SIGINFO 0x53494749
+#endif
+
 #endif  // COMMON_LINUX_ELF_GNU_COMPAT_H_