gnulib-tool: fix build error on macOS with --conditional-dependencies

[Bruno Haible]

A testdir built like this:
  ./gnulib-tool --create-testdir --dir=../testdir --without-tests 
--conditional-dependencies setlocale

fails to build on macOS:

...
rm -f libgnu.a
ar cr libgnu.a    
ar: no archive members specified
usage:  ar -d [-TLsv] archive file ...
        ar -m [-TLsv] archive file ...
        ar -m [-abiTLsv] position archive file ...
        ar -p [-TLsv] archive [file ...]
        ar -q [-cTLsv] archive file ...
        ar -r [-cuTLsv] archive file ...
        ar -r [-abciuTLsv] position archive file ...
        ar -t [-TLsv] archive [file ...]
        ar -x [-ouTLsv] archive [file ...]
make[4]: *** [libgnu.a] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

This patch fixes it.


2019-11-18  Bruno Haible  <address@hidden>

        gnulib-tool: Fix build error on macOS with --conditional-dependencies.
        * gnulib-tool (func_modules_add_dummy): Ignore modules that are
        conditionally enabled.

diff --git a/gnulib-tool b/gnulib-tool
index a41522b..28b5bcb 100755
--- a/gnulib-tool
+++ b/gnulib-tool
@@ -3283,18 +3283,24 @@ func_modules_add_dummy ()
   for module in $modules; do
     func_verify_nontests_module
     if test -n "$module"; then
-      # Extract the value of "lib_SOURCES += ...".
-      for file in `func_get_automake_snippet "$module" | combine_lines |
-                   sed -n -e 's,^lib_SOURCES[   ]*+=\([^#]*\).*$,\1,p'`; do
-        # Ignore .h files since they are not compiled.
-        case "$file" in
-          *.h) ;;
-          *)
-            have_lib_SOURCES=yes
-            break 2
-            ;;
-        esac
-      done
+      if test "$cond_dependencies" = true && func_cond_module_p $module; then
+        # Ignore conditional modules, since they are not guaranteed to
+        # contribute to lib_SOURCES.
+        :
+      else
+        # Extract the value of "lib_SOURCES += ...".
+        for file in `func_get_automake_snippet "$module" | combine_lines |
+                     sed -n -e 's,^lib_SOURCES[         
]*+=\([^#]*\).*$,\1,p'`; do
+          # Ignore .h files since they are not compiled.
+          case "$file" in
+            *.h) ;;
+            *)
+              have_lib_SOURCES=yes
+              break 2
+              ;;
+          esac
+        done
+      fi
     fi
   done
   # Add the dummy module, to make sure the library will be non-empty.