Add some buildsystem tests to Travis

.travis.yml

@@ -131,7 +131,7 @@ matrix:
     #
     # Also change the compiler to GCC 4.7, to ensure we stay compatible
     # with that older version.
-    - env: TEST_SUITES="docomp testinstall" NO_COVERAGE=1 ABI=64 BUILDDIR=build CONFIGFLAGS="--enable-valgrind CC=gcc-4.7 CXX=g++-4.7"
+    - env: TEST_SUITES="docomp testbuildsys testinstall" NO_COVERAGE=1 ABI=64 BUILDDIR=build CONFIGFLAGS="--enable-valgrind CC=gcc-4.7 CXX=g++-4.7"
       addons:
         apt_packages:
           - gcc-4.7
@@ -140,7 +140,7 @@ matrix:
 
     # same as above, but in 32 bit mode, also turn off debugging (see elsewhere in this file for
     # an explanation).
-    - env: TEST_SUITES="docomp testinstall" NO_COVERAGE=1 ABI=32 BUILDDIR=build CONFIGFLAGS=""
+    - env: TEST_SUITES="docomp testbuildsys testinstall" NO_COVERAGE=1 ABI=32 BUILDDIR=build CONFIGFLAGS=""
       addons:
         apt_packages:
           - gcc-multilib

etc/ci.sh

@@ -125,6 +125,36 @@ GAPInput
     git diff --exit-code -- src
     ;;
 
+  testbuildsys)
+    # this test assumes we are doing an out-of-tree build
+    test $BUILDDIR != $SRCDIR
+
+    # test: create garbage *.d and *.lo files in the source dir; these should not
+    # affect the out of tree build, nor should they be removed by `make clean`
+    mkdir -p $SRCDIR/gen/deps/src
+    mkdir -p $SRCDIR/gen/obj/src
+    echo "garbage content !!!" > $SRCDIR/gen/deps/src/bool.c.d
+    echo "garbage content !!!" > $SRCDIR/gen/obj/src/bool.c.lo
+
+    # test: `make clean` works and afterwards we can still `make`
+    make clean
+    make
+
+    # verify our "garbage" files are still there
+    test -f $SRCDIR/gen/deps/src/bool.c.d
+    test -f $SRCDIR/gen/obj/src/bool.c.lo
+
+    # test: `make` should regenerate removed *.d files (and then also regenerate the
+    # corresponding *.lo file, which we verify by overwriting it with garbage)
+    rm gen/deps/src/bool.c.d
+    echo "garbage content !!!" > gen/obj/src/bool.c.lo
+    make
+    test -f gen/deps/src/bool.c.d
+
+    # test: running `make` a second time should produce no output
+    test -z "$(make)"
+    ;;
+
   makemanuals)
     make doc
     make check-manuals