These are libtool libraries meant to be dlopened. They are indicated to libtool by passing -module at link-time.
pkglib_LTLIBRARIES = mymodule.la mymodule_la_SOURCES = doit.c mymodule_la_LDFLAGS = -module
Ordinarily, Automake requires that a library's name start with
lib. However, when building a dynamically loadable module you
might wish to use a "nonstandard" name. Automake will not complain
about such nonstandard names if it knows the library being built is a
libtool module, i.e., if -module explicitly appears in the
_LDFLAGS variable (or in the common
variable when no per-library
_LDFLAGS variable is defined).
AC_SUBST variables are black boxes to Automake since
their values are not yet known when automake is run.
Therefore if -module is set via such a variable, Automake
cannot notice it and will proceed as if the library was an ordinary
libtool library, with strict naming.
mymodule_la_SOURCES is not specified, then it defaults to
the single file mymodule.c (see Default _SOURCES).