AM_PROG_CC_C_O assumption

[Ralf Wildenhues]

CVS Automake assumes that, as soon as per-target compile flags are used,
AM_PROG_CC_C_O is necessary.  Strictly speaking, this assumption is
correct: automake can't tell that there won't be a clash in object file
names.  And the user shouldn't be able to know, since the mapping of
source file names to object file names is an undocumented internal.

However, in practice there are many Makefile.am's out there that use
target_CFLAGS and such, and will definitely never have name conflicts
and/or never possibly be used by a losing compiler.  And as such I think
it is too harsh to have Automake fail hard in this case; giving a
warning would be sufficient IMVHO.

For example, the modular Xorg tree would need hundreds of changes to get
working, most of them actually in Makefile.am's that build only one
object.  A failure due to Automake upgrade will be perceived as an
Automake regression.  (Nevertheless, I intend to bring this issue to
their attention, after hearing your opinion about it.)

Cheers,
Ralf