Propagate muparser include directories to all dependent targets
Propagate muparser include directories to all dependent targets

Use `target_include_directories()` instead of `include_directories()`
to propagate muparser include directory to all targets that link to it.

When consuming muParser in projects via `add_subdirectory()`, one must
also manually add the muParser include directory to the targets due to
the fact that `include_directories()` only adds the directories to
targets in the current CMakeLists file.

CMake documentation at
https://cmake.org/cmake/help/latest/command/include_directories.html
states that `target_include_directories()` should be preferred to add
include directories to individual targets and optionally
propagate/export them to dependents.

Also, the include directory can be added to the muparser library as
`"include"` instead of `"${CMAKE_CURRENT_SOURCE_DIR}/include"` since
relative paths are already interpreted as relative to the current
source directory.

beltoforion/muparser,beltoforion/muparser