Automatically using /usr/local/{include,lib}

[Guillaume Rousse]

Default LDPATH and CPPFLAGS values are empty, making all AC_LIB_CHECK
and AC_HEADER_CHECK unable to detect libraries installed under
/usr/local. Whereas an knowledgeable user will export these variable
before running ./configure, many other will just be unable to build
software properly (and some maintainers may even workaround it by
introducing configure wrappers, I've seen it).

Is there any disadvantage setting default values to thoses variables in
configure.ac, such as:
LDFLAGS=-L/usr/local/lib
CPPFLAGS=-I/usr/local/include

Or is there any other recommended practice ?

-- 
You will realize it's back in season the day that the grocery store runs out
                -- Murphy's Food Laws n°4