(automake-1.16.info)Objects created both with libtool and without

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8.3.9.2 Objects ‘created with both libtool and without’
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Sometimes, the same source file is used both to build a libtool library
and to build another non-libtool target (be it a program or another
library).

   Let’s consider the following ‘Makefile.am’.

     bin_PROGRAMS = prog
     prog_SOURCES = prog.c foo.c ...

     lib_LTLIBRARIES = libfoo.la
     libfoo_la_SOURCES = foo.c ...

(In this trivial case the issue could be avoided by linking ‘libfoo.la’
with ‘prog’ instead of listing ‘foo.c’ in ‘prog_SOURCES’.  But let’s
assume we really want to keep ‘prog’ and ‘libfoo.la’ separate.)

   Technically, it means that we should build ‘foo.$(OBJEXT)’ for
‘prog’, and ‘foo.lo’ for ‘libfoo.la’.  The problem is that in the course
of creating ‘foo.lo’, libtool may erase (or replace) ‘foo.$(OBJEXT)’,
and this cannot be avoided.

   Therefore, when Automake detects this situation it will complain with
a message such as
     object 'foo.$(OBJEXT)' created both with libtool and without

   A workaround for this issue is to ensure that these two objects get
different basenames.  As explained in Note: Renamed Objects, this
happens automatically when per-targets flags are used.

     bin_PROGRAMS = prog
     prog_SOURCES = prog.c foo.c ...
     prog_CFLAGS = $(AM_CFLAGS)

     lib_LTLIBRARIES = libfoo.la
     libfoo_la_SOURCES = foo.c ...

Adding ‘prog_CFLAGS = $(AM_CFLAGS)’ is almost a no-op, because when the
‘prog_CFLAGS’ is defined, it is used instead of ‘AM_CFLAGS’.  However as
a side effect it will cause ‘prog.c’ and ‘foo.c’ to be compiled as
‘prog-prog.$(OBJEXT)’ and ‘prog-foo.$(OBJEXT)’, which solves the issue.