build: improve backtrace support/detection

libexecinfo is used to provide backtrace() on *BSD.  The API is
compatible with glibc's, so this is a "free" improvement.

To improve configure behaviour, the following configure options are
modified/introduced:

* --enable-gcc-rdynamic now defaults to "on" if the compiler is gcc.
  (I sadly wasn't able to find any documentation on the availability of
  this option for llvm, even though at least the version I have
  installed does support it)

* --enable-backtrace has been added.  This behaves as off/auto/on
  switch, i.e. giving either {dis,en}able will result in the requested
  behaviour (or an error if support wasn't found)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

INSTALL.quagga.txt

@@ -31,6 +31,9 @@ not a guarantee of support, merely "we agree that it is broken".)
   OpenBSD ? [info needed on what should work]
   Solaris (modern/supported versions, including OpenSolaris forks) 
 
+On BSD systems, installing libexecinfo is strongly recommended in order
+to get backtrace support.
+
 For further Quagga specific information on 'configure' and build-time
 configuration of the software, please read the Quagga info
 documentation, (doc/quagga.info).  To read the info page included with

configure.ac

@@ -273,7 +273,9 @@ AC_ARG_ENABLE(gcc_ultra_verbose,
 AC_ARG_ENABLE(linux24_tcp_md5,
 [  --enable-linux24-tcp-md5  enable support for old, Linux-2.4 RFC2385 patch])
 AC_ARG_ENABLE(gcc-rdynamic,
-[  --enable-gcc-rdynamic   enable gcc linking with -rdynamic for better backtraces])
+[  --enable-gcc-rdynamic         enable linking with -rdynamic for better backtraces (default if gcc)])
+AC_ARG_ENABLE(backtrace,
+[  --disable-backtrace,          disable crash backtraces (default autodetect)])
 AC_ARG_ENABLE(time-check,
 [  --disable-time-check          disable slow thread warning messages])
 AC_ARG_ENABLE(pcreposix,
@@ -288,8 +290,10 @@ if test x"${enable_gcc_ultra_verbose}" = x"yes" ; then
   CFLAGS="${CFLAGS} -Wpacked -Wpadded"
 fi
 
-if test x"${enable_gcc_rdynamic}" = x"yes" ; then
+if test x"${enable_gcc_rdynamic}" != x"no" ; then
+  if test x"${enable_gcc_rdynamic}" = x"yes" -o x"$COMPILER" = x"GCC"; then
     LDFLAGS="${LDFLAGS} -rdynamic"
+  fi
 fi
 
 if test x"${enable_time_check}" != x"no" ; then
@@ -1566,12 +1570,21 @@ AX_SYS_WEAK_ALIAS
 dnl ---------------------------
 dnl check for glibc 'backtrace'
 dnl --------------------------- 
-AC_CHECK_HEADER([execinfo.h],
-  [AC_CHECK_FUNC([backtrace],
-    [AC_DEFINE(HAVE_GLIBC_BACKTRACE,,[Glibc backtrace])
+if test x"${enable_backtrace}" != x"no" ; then
+  backtrace_ok=no
+  AC_CHECK_HEADER([execinfo.h], [
+    AC_SEARCH_LIBS([backtrace], [execinfo], [
+      AC_DEFINE(HAVE_GLIBC_BACKTRACE,,[Glibc backtrace])
       AC_DEFINE(HAVE_STACK_TRACE,,[Stack symbol decoding])
+      backtrace_ok=yes
+    ],, [-lm])
   ])
-])
+
+  if test x"${enable_backtrace}" = x"yes" -a x"${backtrace_ok}" = x"no"; then
+    dnl user explicitly requested backtrace but we failed to find support
+    AC_MSG_FAILURE([failed to find backtrace support])
+  fi
+fi
 
 dnl -----------------------------------------
 dnl check for malloc mallinfo struct and call

doc/install.texi

@@ -96,6 +96,16 @@ installed.  They can be excluded from build with this option, which will
 minimally decrease compile time and overhead.  They can always be built and
 executed at a later time by running @command{make check} in the @file{tests/}
 subdirectory, even if they're excluded from build.
+@item --enable-gcc-rdynamic
+Pass the @command{-rdynamic} option to the linker driver.  This is in most
+cases neccessary for getting usable backtraces.  This option defaults to on
+if the compiler is detected as gcc, but giving an explicit enable/disable is
+suggested.
+@item --enable-backtrace
+Controls backtrace support for the crash handlers. This is autodetected by
+default. Using the switch will enforce the requested behaviour, failing with
+an error if support is requested but not available.  On BSD systems, this
+needs libexecinfo, while on glibc support for this is part of libc itself.
 @end table
 
 You may specify any combination of the above options to the configure