Unix: compatibility with platforms where backtrace() is not in libc.

Also, don't depend on backtrace() when built as libslvs; it is not
the responsbility of the library to use it.

CMakeLists.txt

@@ -187,6 +187,7 @@ elseif(APPLE)
     find_package(Freetype REQUIRED)
     find_library(APPKIT_LIBRARY AppKit REQUIRED)
 else() # Linux and compatible systems
+    find_package(Backtrace)
     find_package(SpaceWare)
 
     # Use freedesktop's pkg-config to locate everything.

src/CMakeLists.txt

@@ -216,6 +216,11 @@ if(SPACEWARE_FOUND)
         ${SPACEWARE_LIBRARIES})
 endif()
 
+if(Backtrace_FOUND)
+    target_link_libraries(solvespace
+        ${Backtrace_LIBRARIES})
+endif()
+
 if(APPLE)
     foreach(lib ${platform_BUNDLED_LIBS})
         get_filename_component(name ${lib} NAME)

src/platform/unixutil.cpp

@@ -31,6 +31,7 @@ void assert_failure(const char *file, unsigned line, const char *function,
     fprintf(stderr, "File %s, line %u, function %s:\n", file, line, function);
     fprintf(stderr, "Assertion '%s' failed: ((%s) == false).\n", message, condition);
 
+#ifndef LIBRARY
     static void *ptrs[1024] = {};
     size_t nptrs = backtrace(ptrs, sizeof(ptrs) / sizeof(ptrs[0]));
     char **syms = backtrace_symbols(ptrs, nptrs);
@@ -45,6 +46,7 @@ void assert_failure(const char *file, unsigned line, const char *function,
             fprintf(stderr, "%2zu: %p\n", i, ptrs[i]);
         }
     }
+#endif
 
     abort();
 }