Support custom debug dirs

[jtojnar]

On platforms that do not use FHS like NixOS or GNU Guix, the build-id directories are not under /usr/lib/debug.

Let’s add --with-separate-debug-dir configure flag so that the path can be changed. The same flag is supported by gdb

And just like gdb, let’s make it accept multiple debug directories separated by colons. This is useful for example when using dwarffs in addition to debug data installed using distribution’s package manager.

[jtojnar]

Ideally, we would be able to just use -D flag here but I tried the following

--- a/Makefile.am
+++ b/Makefile.am
@@ -144,12 +144,9 @@ libbacktrace_elf_for_test_la_LIBADD = $(BACKTRACE_FILE) elf_for_test.lo \
 	$(VIEW_FILE) $(ALLOC_FILE)
 
 elf_for_test.c: elf.c
-	SEARCH='^#define BUILD_ID_DIR.*$$'; \
-	REPLACE='\0\n#undef SYSTEM_DEBUG_DIR\n#define SYSTEM_DEBUG_DIR "$(TEST_DEBUG_DIR)"'; \
-	$(SED) "s%$$SEARCH%$$REPLACE%" \
-		$< \
-		> $@.tmp
-	mv $@.tmp $@
+	cp $< $@
+
+libbacktrace_elf_for_test_la_CFLAGS = -USYSTEM_DEBUG_DIR -DSYSTEM_DEBUG_DIR=\"$(TEST_DEBUG_DIR)\"
 
 endif HAVE_OBJCOPY_DEBUGLINK
 endif HAVE_ELF

without success and autotools are just too confusing to me.

libbacktrace_elf_for_test_la_CFLAGS does not seem to affect the compilation of elf_for_test.lo (it still gets -DSYSTEM_DEBUG_DIR from AM_CFLAGS) and it only appears when linking the objects into libbacktrace_elf_for_test.la:

cp elf.c elf_for_test.c
/nix/store/4xw8n979xpivdc46a9ndcvyhwgif00hz-bash-5.1-p16/bin/bash ./libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I.    -funwind-tables -frandom-seed=elf_for_test.lo -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition -Wmissing-format-attribute -Wcast-qual  -DSYSTEM_DEBUG_DIR=\"/nix/store/dkpyi2cxvjxk74l2pjighshbzncy2hmy-libbacktrace-unstable-2022-12-16/lib/debug\" -g -O2 -c -o elf_for_test.lo elf_for_test.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -funwind-tables -frandom-seed=elf_for_test.lo -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition -Wmissing-format-attribute -Wcast-qual -DSYSTEM_DEBUG_DIR=\"/nix/store/dkpyi2cxvjxk74l2pjighshbzncy2hmy-libbacktrace-unstable-2022-12-16/lib/debug\" -g -O2 -c elf_for_test.c  -fPIC -DPIC -o .libs/elf_for_test.o
/nix/store/4xw8n979xpivdc46a9ndcvyhwgif00hz-bash-5.1-p16/bin/bash ./libtool  --tag=CC   --mode=link gcc -USYSTEM_DEBUG_DIR -DSYSTEM_DEBUG_DIR=\"/build/libbacktrace/usr/lib/debug\" -g -O2   -o libbacktrace_elf_for_test.la  libbacktrace_elf_for_test_la-atomic.lo libbacktrace_elf_for_test_la-dwarf.lo libbacktrace_elf_for_test_la-fileline.lo libbacktrace_elf_for_test_la-posix.lo libbacktrace_elf_for_test_la-print.lo libbacktrace_elf_for_test_la-sort.lo libbacktrace_elf_for_test_la-state.lo backtrace.lo simple.lo elf_for_test.lo mmapio.lo mmap.lo 
libtool: link: ar cr .libs/libbacktrace_elf_for_test.a .libs/libbacktrace_elf_for_test_la-atomic.o .libs/libbacktrace_elf_for_test_la-dwarf.o .libs/libbacktrace_elf_for_test_la-fileline.o .libs/libbacktrace_elf_for_test_la-posix.o .libs/libbacktrace_elf_for_test_la-print.o .libs/libbacktrace_elf_for_test_la-sort.o .libs/libbacktrace_elf_for_test_la-state.o .libs/backtrace.o .libs/simple.o .libs/elf_for_test.o .libs/mmapio.o .libs/mmap.o 
libtool: link: ranlib .libs/libbacktrace_elf_for_test.a