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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