src: Makefile.am: fix checkheader with C++ headers

The checkheaders mechanism wasn't compatible with .hpp headers, and even more, because of substitution failure, the .hpp headers were deleted when make distclean was run.

Separate .h and .hpp headers in the pluginsinclude_HEADER variable to be able to associate separate target variables for them, in particular for the checkheader feature.

Indeed, POSIX make implementations are perfectly able to create macro substitution and merge multiple macros together, but they are not able to filter or substract in a general way (only one-item substraction works) from an existing macro, so the way this should be done is by splitting the source items and merge them where needed.

This MR uses the newly-separated pluginsinclude_HEADERS.h and pluginsinclude_HEADERS.hpp variables to supply different variants for the CHECKCOMPILE and CHECK_SOURCE_EXT variable depending on the file type.