build: Rework config=coverage to be more robust

Use find -exec instead of xargs to work around differences between xargs on OSX
and Linux.

Use -b option of gcov - for some reason gcov on Travis can't find .gcno files
otherwise (old version?).

And finally enable config=coverage again.

.travis.yml

@@ -7,5 +7,5 @@ env:
   - DEFINES=standard
   - DEFINES=PUGIXML_WCHAR_MODE
   - DEFINES=PUGIXML_COMPACT
-script: make test defines=$DEFINES -j2
+script: make test defines=$DEFINES config=coverage -j2
 after_success: bash <(curl -s https://codecov.io/bash)

Makefile

@@ -54,10 +54,10 @@ all: $(EXECUTABLE)
 
 ifeq ($(config),coverage)
 test: $(EXECUTABLE)
-	-@find $(BUILD) -name '*.gcda' | xargs rm
+	-@find $(BUILD) -name '*.gcda' -exec rm {} +
 	./$(EXECUTABLE)
-	@gcov -b -c $(BUILD)/src/pugixml.cpp.gcda | sed -e '/./{H;$!d;}' -e 'x;/pugixml.cpp/!d;'
-	@ls *.gcov | grep -v pugixml.cpp.gcov | xargs rm
+	@gcov -b -o=$(BUILD)/src pugixml.cpp.gcda | sed -e '/./{H;$!d;}' -e 'x;/pugixml.cpp/!d;'
+	@find . -name '*.gcov' -and -not -name 'pugixml.cpp.gcov' -exec rm {} +
 else
 test: $(EXECUTABLE)
 	./$(EXECUTABLE)