demote missing gtest to a buildtime warning

We just need it for unit tests and our debian/rules file actually skips
calling them if nocheck is given… but this fails anyhow as we declared a
hard-dependency on it. Demoting the error to a warning in configuration
and adding a test in the 'make test' path with a friendly message allows
nocheck to be useful again.

(Running unit tests is fully encouraged of course, but bootstrappers and
 co do not need to be burdened with this stuff)

configure.ac

@@ -91,7 +91,7 @@ AC_CHECK_LIB(curl, curl_easy_init,
 
 AC_LANG_PUSH([C++])
 AC_CHECK_HEADER(gtest/gtest.h,,
-     AC_MSG_ERROR([failed: I need gtest to build tests]),
+     AC_MSG_WARN([failed: I need gtest (packaged as libgtest-dev) for unit testing]),
 )
 AC_LANG_POP([C++])
 

test/libapt/makefile

@@ -8,6 +8,7 @@ APT_DOMAIN=none
 include ../../buildlib/defaults.mak
 
 .PHONY: test
+ifeq (file-okay,$(shell $(CC) -M gtest_runner.cc >/dev/null 2>&1 && echo 'file-okay'))
 test: $(BIN)/gtest$(BASENAME)
 	MALLOC_PERTURB_=21 MALLOC_CHECK_=2 LD_LIBRARY_PATH=$(LIB) $(BIN)/gtest$(BASENAME)
 
@@ -71,3 +72,11 @@ $(LIB)/gtest.a: $(OBJ)/gtest-all.o
 	echo Building static library $@
 	-rm -f $@
 	$(AR) $(ARFLAGS) $@ $^
+
+else
+test:
+	@echo "APT uses Googles C++ testing framework for its unit tests"
+	@echo "On Debian systems this is available in the 'libgtest-dev' package."
+	@echo "Please install it before attempting to run the unit tests."
+	exit 100
+endif