This improves usability of sdbus-c++ in downstream CMake projects. CMake options now have `SDBUSCPP_` prefix so potential conflicts with downstream options/variables are minimized. Also, all configurable options and variables are placed in a single place in the root CMake file.
This patch is required if EXPAT library is installed in non-standard location.
Without 'target_include_directories' cmake will find EXPAT library:
...
-- Found EXPAT: .../lib/libexpat.a (found version "2.2.10")
...
But 'xml.cpp' compilation will fail with error:
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tools/xml2cpp-codegen/xml.cpp:7:10: fatal error: expat.h: No such file or directory
7 | #include <expat.h>
...