build: Adjust tests to Automake 1.12.

* Makefile.am (TESTS_ENVIRONMENT): Remove `guile' invocation.  Add
  semicolons, and export the variables as shown in the Automake 1.12
  manual.
  (LOG_COMPILER): New variable, with `guile' invocation.

.gitignore

@@ -35,3 +35,4 @@ config.cache
 /po/stamp-po
 /po/guix.pot
 /guix-build
+/tests/*.trs

Makefile.am

@@ -49,8 +49,11 @@ TESTS =						\
   tests/packages.scm
 
 TESTS_ENVIRONMENT =								\
-  NIXPKGS="$(NIXPKGS)"							\
-  GUILE_LOAD_COMPILED_PATH="$(top_builddir):$$GUILE_LOAD_COMPILED_PATH"	\
+  NIXPKGS="$(NIXPKGS)";								\
+  GUILE_LOAD_COMPILED_PATH="$(top_builddir):$$GUILE_LOAD_COMPILED_PATH";	\
+  export NIXPKGS GUILE_LOAD_COMPILED_PATH;
+
+LOG_COMPILER =					\
   $(GUILE) --no-auto-compile -L "$(top_srcdir)"
 
 EXTRA_DIST = build-aux/config.rpath $(TESTS)