Update backward-cpp library (for M1 Macs)

[blake-riley]

Hi Paul,

Yoshitaka's Homebrew script is fantastic. Coot pre-1.0 almost builds on the fancy, new M1 processors.

At the moment, compilation on Apple Silicon fails on utils/backward.hpp. A short patch (see bombela/backward-cpp#201) is all that's blocking compilation.

diff --git a/utils/backward.hpp b/utils/backward.hpp
index 195ca8f2d..e661ce7b6 100644
--- a/utils/backward.hpp
+++ b/utils/backward.hpp
@@ -4157,7 +4157,11 @@ class SignalHandling {
 #elif defined(__arm__)
     error_addr = reinterpret_cast<void *>(uctx->uc_mcontext.arm_pc);
 #elif defined(__aarch64__)
-    error_addr = reinterpret_cast<void *>(uctx->uc_mcontext.pc);
+    #if defined(__APPLE__)
+      error_addr = reinterpret_cast<void *>(uctx->uc_mcontext->__ss.__pc);
+    #else
+      error_addr = reinterpret_cast<void *>(uctx->uc_mcontext.pc);
+    #endif
 #elif defined(__mips__)
     error_addr = reinterpret_cast<void *>(
         reinterpret_cast<struct sigcontext *>(&uctx->uc_mcontext)->sc_pc);

You might wish to:

1. Apply this diff, or
2. Bump your version of backward-cpp to the latest release v1.6, which includes this patch

Cheers for Coot!
Blake

[pemsley]

Hi Blake, I had intended for backward to be conditionally compiled (default not) and changed the configure script to do so in e633e2d. I will check out the newer version of backward and see how that works. Thanks.