System includes for synthetic database

[K-ballo]

The tool will synthesize a compilation database if none is given. This db is detached from the system and thus doesn't know how to locate system includes.

mrdocs/src/lib/MrDocsSettingsDB.cpp

Line 69 in e2c8aac

cmds.emplace_back("clang");

System includes are located by executing the compiler (only meaningful for gcc and clang, msvc does differently)

mrdocs/src/tool/ToolCompilationDatabase.cpp

Line 104 in e2c8aac

auto const defaultIncludePaths = getCompilersDefaultIncludeDir(

Ideally these paths would be resolved without executing any external processes. Otherwise, the synthetic database would have to know about a real compiler in the system.

This blocks #1042 , as some of the demos rely on this functionality.