external: update backward-cpp to v1.4

rename the directory name to backward-cpp

Closes: #803
Signed-off-by: Shengjing Zhu <i@zhsj.me>

external/backward-cpp/test_package/conanfile.py

@@ -0,0 +1,14 @@
+from conans import ConanFile, CMake
+import os
+
+class TestBackward(ConanFile):
+    settings = 'os', 'compiler', 'build_type', 'arch'
+    generators = 'cmake'
+
+    def build(self):
+        cmake = CMake(self)
+        cmake.configure(defs={'CMAKE_VERBOSE_MAKEFILE': 'ON'})
+        cmake.build()
+
+    def test(self):
+        self.run(os.path.join('.', 'bin', 'example'))