[master] Use explicit name of the first prerequisite in ext/gtest.

The $< variable is not defined in explicit rules on FreeBSD10 and causes
Kea build to fail. This change was reviewed and agreed on jabber.

ext/gtest/Makefile.am

@@ -7,4 +7,4 @@ libgtest_a_CXXFLAGS = $(GTEST_INCLUDES) $(AM_CXXFLAGS)
 nodist_libgtest_a_SOURCES = gtest-all.cc
 
 gtest-all.cc: $(GTEST_SOURCE)/src/gtest-all.cc
-	cp -p $< $@
+	cp -p $(GTEST_SOURCE)/src/gtest-all.cc $@