Makefile.am and the _LDADD option

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Hello,

In my Makefile.am I try this:
xmlparsefile_LDFLAGS = `pkg-config libxml-2.0 --libs-only-L`
xmlparsefile_LDADD = `pkg-config libxml-2.0 --libs-only-l`
Then it is refused:
$ autoreconf
src/Makefile.am:6: linker flags such as `--libs-only-l`' belong in 
`xmlparsefile_LDFLAGS
autoreconf: automake failed with exit status: 1

So there is an other solution:
-edit the configure.ac file:
PKG_CHECK_MODULES([LIBXML], [libxml-2.0 >= 2.7.8])
-edit the Makefile.am file:
xmlparsefile_LDADD = $(LIBXML_LIBS)
xmlparsefile_CFLAGS = -Wall -Wextra $(LIBXML_CFLAGS)
An now, it is accepted.

But I am wondering this: why the first solution doesn't works?

Thank you.