(automake-1.16.info)Per-Object Flags

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27.8 Per-Object Flags Emulation
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     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 requires?  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,
for instance ‘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’, 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.