libbacktrace: Allow configuring debug dir

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:

https://github.com/bminor/binutils-gdb/blob/095f84c7e3cf85cd68c657c46b80be078f336bc9/gdb/configure.ac#L113-L115

Makefile.am

@@ -33,7 +33,8 @@ ACLOCAL_AMFLAGS = -I config
 
 AM_CPPFLAGS =
 
-AM_CFLAGS = $(EXTRA_FLAGS) $(WARN_FLAGS) $(PIC_FLAG)
+AM_CFLAGS = $(EXTRA_FLAGS) $(WARN_FLAGS) $(PIC_FLAG) \
+	-DSYSTEM_DEBUG_DIR=\"$(SEPARATE_DEBUG_DIR)\"
 
 include_HEADERS = backtrace.h backtrace-supported.h
 
@@ -132,7 +133,7 @@ libbacktrace_noformat_la_DEPENDENCIES = $(libbacktrace_noformat_la_LIBADD)
 if HAVE_ELF
 if HAVE_OBJCOPY_DEBUGLINK
 
-TEST_BUILD_ID_DIR=$(abs_builddir)/usr/lib/debug/.build-id/
+TEST_DEBUG_DIR=$(abs_builddir)/usr/lib/debug
 
 check_LTLIBRARIES += libbacktrace_elf_for_test.la
 
@@ -141,8 +142,8 @@ libbacktrace_elf_for_test_la_LIBADD = $(BACKTRACE_FILE) elf_for_test.lo \
 	$(VIEW_FILE) $(ALLOC_FILE)
 
 elf_for_test.c: elf.c
-	SEARCH='^#define SYSTEM_BUILD_ID_DIR.*$$'; \
-	REPLACE="#define SYSTEM_BUILD_ID_DIR \"$(TEST_BUILD_ID_DIR)\""; \
+	SEARCH='^#define BUILD_ID_DIR.*$$'; \
+	REPLACE='\0\n#undef SYSTEM_DEBUG_DIR\n#define SYSTEM_DEBUG_DIR "$(TEST_DEBUG_DIR)"'; \
 	$(SED) "s%$$SEARCH%$$REPLACE%" \
 		$< \
 		> $@.tmp
@@ -458,7 +459,7 @@ endif HAVE_OBJCOPY_DEBUGLINK
 
 %_buildid: %
 	./install-debuginfo-for-buildid.sh \
-	  "$(TEST_BUILD_ID_DIR)" \
+	  "$(TEST_DEBUG_DIR)/.build-id" \
 	  $<
 	$(OBJCOPY) --strip-debug $< $@
 

configure.ac

@@ -67,6 +67,14 @@ AM_MAINTAINER_MODE
 AC_ARG_WITH(target-subdir,
 [  --with-target-subdir=SUBDIR      Configuring in a subdirectory for target])
 
+AC_ARG_WITH(separate-debug-dir,
+[  --with-separate-debug-dir=DEBUGDIR      Look for global separate debug info in this path @<:@LIBDIR/debug@:>@],
+[separate_debug_dir=$withval],
+[separate_debug_dir=$libdir/debug])
+
+SEPARATE_DEBUG_DIR=$separate_debug_dir
+AC_SUBST(SEPARATE_DEBUG_DIR)
+
 # We must force CC to /not/ be precious variables; otherwise
 # the wrong, non-multilib-adjusted value will be used in multilibs.
 # As a side effect, we have to subst CFLAGS ourselves.

elf.c

@@ -856,7 +856,7 @@ elf_readlink (struct backtrace_state *state, const char *filename,
     }
 }
 
-#define SYSTEM_BUILD_ID_DIR "/usr/lib/debug/.build-id/"
+#define BUILD_ID_DIR "/.build-id/"
 
 /* Open a separate debug info file, using the build ID to find it.
    Returns an open file descriptor, or -1.
@@ -870,7 +870,7 @@ elf_open_debugfile_by_buildid (struct backtrace_state *state,
 			       backtrace_error_callback error_callback,
 			       void *data)
 {
-  const char * const prefix = SYSTEM_BUILD_ID_DIR;
+  const char * const prefix = SYSTEM_DEBUG_DIR BUILD_ID_DIR;
   const size_t prefix_len = strlen (prefix);
   const char * const suffix = ".debug";
   const size_t suffix_len = strlen (suffix);