Add retain basic implementation on GNU/Linux

Retain is saved in file and backup of this file is done every time.
File isn't saved on every PLC cycle to avoid high I/O load.
By default, retain is saved every second and on PLC stop/shutdown.
It's possible to override retain flush period by using CFLAGS option
in project's settings. For example, for 0.5 seconds period
use "-DFILE_RETAIN_SAVE_PERIOD_S=0.5".

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#! gmake

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yml := ../../yml2

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ysl2files := $(wildcard *.ysl2)

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xsltfiles := $(patsubst %.ysl2, %.xslt, $(ysl2files))

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all:$(xsltfiles)

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%.xslt: %.ysl2 yslt_noindent.yml2

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	$(yml)/yml2c -I $(yml) $< -o $@.tmp

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	xmlstarlet fo $@.tmp > $@

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	rm $@.tmp

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clean:

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	rm -f $(xsltfiles)