make/checksrc: use $srcdir, not $top_srcdir

docs/examples/Makefile.am

@@ -63,4 +63,4 @@ include Makefile.inc
 all: $(check_PROGRAMS)
 
 checksrc:
-	@PERL@ $(top_srcdir)/lib/checksrc.pl $(top_srcdir)/docs/examples/*.c
+	@PERL@ $(top_srcdir)/lib/checksrc.pl $(srcdir)/*.c

tests/libtest/Makefile.am

@@ -117,4 +117,4 @@ libhostname_la_LIBADD =
 libhostname_la_DEPENDENCIES =
 
 checksrc:
-	@PERL@ $(top_srcdir)/lib/checksrc.pl $(top_srcdir)/tests/libtest/*.c
+	@PERL@ $(top_srcdir)/lib/checksrc.pl $(srcdir)/*.c

tests/server/Makefile.am

@@ -63,4 +63,4 @@ include Makefile.inc
 EXTRA_DIST = base64.pl Makefile.inc CMakeLists.txt
 
 checksrc:
-	@PERL@ $(top_srcdir)/lib/checksrc.pl $(top_srcdir)/tests/libtest/*.c
+	@PERL@ $(top_srcdir)/lib/checksrc.pl $(srcdir)/*.c

tests/unit/Makefile.am

@@ -72,7 +72,7 @@ AM_CPPFLAGS += -DCURL_STATICLIB -DUNITTESTS
 include Makefile.inc
 
 checksrc:
-	@PERL@ $(top_srcdir)/lib/checksrc.pl $(top_srcdir)/tests/unit/*.c
+	@PERL@ $(top_srcdir)/lib/checksrc.pl $(srcdir)/*.c
 
 if BUILD_UNITTESTS
 noinst_PROGRAMS = $(UNITPROGS)