H.2.2 Build Directories

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 ‘$(srcdir)/’. It’s safer to quote the source directory name, in case it contains characters that are special to the shell. Because ‘$(srcdir)’ is expanded by Make, single-quoting works and is safer than double-quoting. For example:

time.info: time.texinfo
        $(MAKEINFO) '$(srcdir)/time.texinfo'