How about this proposed new entry for the FAQ?
26.7 Per-Object Flags Emulation
===============================
One of my source files needs to be compiled with different flags. How
do I do?
Automake supports per-program and per-library compilation flags (see
*Note Program and Library Variables:: and *Note Flag Variables
Ordering::). With this you can define compilation flags that apply to
all files compiled for a target. For instance in
bin_PROGRAMS = foo
foo_SOURCES = foo.c foo.h bar.c bar.h main.c
foo_CFLAGS = -some -flags
`foo-foo.o', `foo-bar.o', and `foo-main.o' will all be compiled with
`-some -flags'. (If you wonder about the names of these object files,
see *Note renamed objects::.) Note that `foo_CFLAGS' gives the flags
to use when compiling all the C sources of the _program_ `foo', it has
nothing to do with `foo.c' or `foo-foo.o' specifically.
What if `foo.c' needs to be compiled into `foo.o' using some
specific flags, that none of the other files require? Obviously
per-program flags are not directly applicable here. Something like
per-object flags are expected, i.e., flags that would be used only when
creating `foo-foo.o'. Automake does not support that, however this is
easy to simulate using a library that contains only that object, and
compiling this library with per-library flags.
bin_PROGRAMS = foo
foo_SOURCES = bar.c bar.h main.c
foo_CFLAGS = -some -flags
foo_LDADD = libfoo.a
noinst_LIBRARIES = libfoo.a
libfoo_a_SOURCES = foo.c foo.h
libfoo_a_CFLAGS = -some -other -flags
Here `foo-bar.o' and `foo-main.o' will all be compiled with `-some
-flags', while `libfoo_a-foo.o' will be compiled using `-some -other
-flags'. Eventually, all three objects will be linked to form `foo'.
This trick can also be achieved using Libtool convenience libraries,
i.e., `noinst_LTLIBRARIES = libfoo.la' (*note Libtool Convenience
Libraries::).
Another tempting idea to implement per-object flags is to override
the compile rules `automake' would output for these files. Automake
will not define a rule for a target you have defined, so you could
think about defining the `foo-foo.o: foo.c' rule yourself. We
recommend against this, because this is error prone. For instance if
you add such a rule to the first example, it will break the day you
decide to remove `foo_CFLAGS' (because `foo.c' will then be compiled as
`foo.o' instead of `foo-foo.o', see *Note renamed objects::). Also in
order to support dependency tracking, the two `.o'/`.obj' extensions,
and all the other flags variables involved in a compilation, you will
end up modifying a copy of the rule previously output by `automake' for
this file. If a new release of Automake generates a different rule,
your copy will need to be updated by hand.
--
Alexandre Duret-Lutz
