improved plugin & associated makefile for recent target toolchains

[gwbres]

Hey guys,

I faced issues when compiling your tree with more recent target toolchains (the ones embedded in the tree are 5.4+). I know you don't propose recent toolchains directly, but this will help in later upgrades & does not break your code.

I found that the right way to include the plugin development headers is now #include <gcc-plugin.h>

For portability reason, I introduce a -DTARGET_CXX_VERSION= flag, which in the original case will equal 5, but for instance might equal 8 if I point to a newer toolchain.

That is why I modify #include <plugin.h> in nexmon.c if necessary

In newer toolchains the 'attribute_specs' structure has new attributes, which causes the previous construction to fail because the ordering is no longer respected. Therefore I also propose a reordering, if necessary