You can support compiling a software package for several architectures simultaneously from the same copy of the source code. The object files for each architecture are kept in their own directory.
To support doing this, make uses the VPATH variable to
find the files that are in the source directory. GNU Make
can do this. Most other recent make programs can do this as
well, though they may have difficulties and it is often simpler to
recommend GNU make (see VPATH and Make). Older
make programs do not support VPATH; when using them, the
source code must be in the same directory as the object files.
If you are using GNU Automake, the remaining details in this
section are already covered for you, based on the contents of your
Makefile.am. But if you are using Autoconf in isolation, then
supporting VPATH requires the following in your
Makefile.in:
srcdir = @srcdir@
VPATH = @srcdir@
Do not set VPATH to the value of another variable (see Variables listed in VPATH.
configure substitutes the correct value for srcdir when
it produces Makefile.
Do not use the make variable $<, which expands to the
file name of the file in the source directory (found with VPATH),
except in implicit rules. (An implicit rule is one such as ‘.c.o’,
which tells how to create a .o file from a .c file.) Some
versions of make do not set $< in explicit rules; they
expand it to an empty value.
Instead, Make command lines should always refer to source files by prefixing them with ‘$(srcdir)/’. For example:
time.info: time.texinfo
$(MAKEINFO) '$(srcdir)/time.texinfo'