AM_PROG_CC_C_O and subdir-objects

[Lars Hecking]

 Why does the 'subdir-objects' option require AM_PROG_CC_C_O?

 I think I figured it out - the generated compile rules do something like
 $CC -c -o dir/file.o dir/file.c, is that correct? (I don't have a working
 test case, this came up on a mailing list earlier.)

 This is kind of obscure, though, and the connection should be documented.