conanfile: set compatibility_cppstr = False.

The Catch libraries have different API/ABI depending on the c++
standard they are compiled with. For example, the following function
isn't in the binary when compiled with C++14, only with C++17 or later:

StringMaker<std::string_view>::convert(std::string_view str);

By default, Conan is allowed to serve Catch libraries compiled in C++14
into a project using C++17/20, potentially causing linker errors
because of missing symbols. This PR overrides this default behaviour:
the C++ standard of the Catch library will exactly match the one of
the requiring project (building Catch from source if necessary).

conanfile.py

@@ -19,6 +19,7 @@ class CatchConan(ConanFile):
     license = "BSL-1.0"
     version = "latest"
     settings = "os", "compiler", "build_type", "arch"
+    extension_properties = {"compatibility_cppstd": False}
 
     options = {
         "shared": [True, False],