Removes the use of backtrace as musl libc does not support it.
Patch is upstreamed and should not be needed after FreeCAD v0.20.
--- ./src/App/Application.cpp
+++ ./src/App/Application.cpp
@@ -1589,6 +1589,7 @@
 // This function produces a stack backtrace with demangled function & method names.
 void printBacktrace(size_t skip=0)
 {
+#if BACKTRACE_PRESENT
     void *callstack[128];
     size_t nMaxFrames = sizeof(callstack) / sizeof(callstack[0]);
     size_t nFrames = backtrace(callstack, nMaxFrames);
@@ -1619,6 +1620,9 @@
     }
 
     free(symbols);
+#else
+    std:cerr << "FreeCAD has not been compiled with backtrace support and thus some debug information is not available" << std::endl;
+#endif
 }
 #endif
 
--- ./cMake/ConfigureChecks.cmake
+++ ./cMake/ConfigureChecks.cmake
@@ -62,4 +62,6 @@
 SET(HAVE_QT_KEYPAD_DEFINE 1)
 SET(HAVE_QWIDGET_SHOWFULLSCREEN 1)
 
-
+IF(Backtrace_FOUND)
+    ADD_COMPILE_DEFINITIONS(BACKTRACE_PRESENT)
+ENDIF(Backtrace_FOUND)