Automake and external conditional sources - makefile

Question

How to properly define external conditional sources in a Makefile.am file?
As an example, in GNU MPFR, we currently have in src/Makefile.am:
if MINI_GMP
nodist_include_HEADERS += $(mini_gmp_path)/mini-gmp.h
noinst_LTLIBRARIES = libminigmp.la
nodist_libminigmp_la_SOURCES = $(mini_gmp_path)/mini-gmp.h $(mini_gmp_path)/mini-gmp.c
libmpfr_la_LIBADD += libminigmp.la
endif
where MINI_GMP is true when mini_gmp_path is defined. The goal is to optionally build MPFR against a libminigmp library (mini-gmp), which is itself built with the autotools machinery (same compiler, same options, etc. as MPFR).
Everything seems to work well, except when one does not want to build against mini-gmp. In this case, the generated configure script fails with:
configure: creating ./config.status
config.status: creating Makefile
config.status: creating mpfr.pc
config.status: creating doc/Makefile
config.status: creating src/Makefile
config.status: creating tests/Makefile
config.status: creating tune/Makefile
config.status: creating src/mparam.h
config.status: creating tools/bench/Makefile
config.status: executing depfiles commands
config.status: error: in `/home/vlefevre/software/mpfr':
config.status: error: Something went wrong bootstrapping makefile fragments
for automatic dependency tracking. Try re-running configure with the
'--disable-dependency-tracking' option to at least be able to build
the package (albeit without support for automatic dependency tracking).
See `config.log' for more details
and in the config.log file:
config.status:19572: executing depfiles commands
config.status:19647: cd src && sed -e '/# am--include-marker/d' Makefile | make -f - am--depfiles
/bin/mkdir: cannot create directory '/.deps': Permission denied
make: *** [/tmp/GmlvxQJQ:720: /.deps/mini-gmp.Plo] Error 1
The reason is that the generated src/Makefile file contains things like $(mini_gmp_path)/$(DEPDIR)/mini-gmp.Plo while mini_gmp_path is not defined. But since one does not want to build against mini-gmp, such information about mini-gmp.Plo is incorrect, i.e. mini-gmp files should completely be ignored for automatic dependency tracking when MINI_GMP is false.
As a workaround, we currently define mini_gmp_path to . in configure.ac when one does not build against mini-gmp, so that the $(mini_gmp_path)/$(DEPDIR) directory exists, avoiding a configure failure. But this is not a clean solution, which might interfere with mini-gmp.{c,h} files that could be added in the MPFR source tree. The solution should just get rid of mini-gmp.Plo entirely.

Answer 1

I don't think incorporating external sources into your build is a supported use case. It certainly does not make much sense to me. Nevertheless, possible alternatives include
Bundle the minigmp sources with your project (their use can still be optional). This has the additional advantage that you can be sure you provide a compatible version.
Add standard make rules to create the minigmp sources in some existing directory inside the project by copying them from $(mini_gmp_path). You may also want to list the minigmp sources (in the internal directory) among your BUILT_SOURCES.
if MINI_GMP
noinst_LTLIBRARIES = libminigmp.la
libmpfr_la_LIBADD += libminigmp.la
nodist_libminigmp_la_SOURCES = minigmp/mini-gmp.h minigmp/mini-gmp.c
BUILT_SOURCES = $(nodist_libminigmp_la_SOURCES)
# Are you want the mini-gmp header to be installed?
nodist_include_HEADERS += minigmp/mini-gmp.h
else
noinst_LTLIBRARIES =
BUILT_SOURCES =
endif
minigmp/mini-gmp.c: $(mini_gmp_path)/mini-gmp.c
cp $< $#
minigmp/mini-gmp.h: $(mini_gmp_path)/mini-gmp.h
cp $< $#
Automake should not subject such rules to the same level of analysis that it does the variables that are meaningful to it.
Put the onus on the project builder to build and install a compatible version of minigmp. Link against that.