build,plugins: update build rules
We try to combine in common envvars the compiler and linker flags shared by the different components, and where possible, also re-using the implicit AM_CFLAGS and AM_LDFLAGS variables that automake provides, and which apply to all objects being built in the same Makefile.am. The plugins build rules are also updated so that whenever a plugin has some testable 'helpers', these are compiled into a noinst library and then the new library included by both the plugin and the tester program. This avoids multiple recompilations of the same sources for different objects.
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