build: fix automake non standard library name

Otherwise, we will run into this error:

> thunar/Makefile.am:216: error: 'thunar.a' is not a standard library name

.gitignore

@@ -163,3 +163,4 @@ gtk-doc.m4
 test-driver
 m4/*
 !m4/introspection.m4
+thunar/libthunar.a

docs/reference/thunar/Makefile.am

@@ -64,7 +64,7 @@ GTKDOC_CFLAGS =	 							\
 GTKDOC_LIBS =  								\
 	$(GTK_LIBS)							\
 	$(top_builddir)/thunarx/libthunarx-$(THUNARX_VERSION_API).la	\
-	$(top_builddir)/thunar/thunar.a					\
+	$(top_builddir)/thunar/libthunar.a				\
 	$(EXO_LIBS)							\
 	$(GIO_LIBS)							\
 	$(GTHREAD_LIBS)							\

thunar/Makefile.am

@@ -213,9 +213,9 @@ thunar_SOURCES =							\
 	thunar-window.c							\
 	thunar-window.h
 
-noinst_LIBRARIES = thunar.a
-thunar_a_SOURCES =
-thunar_a_LIBADD = 							\
+noinst_LIBRARIES = libthunar.a
+libthunar_a_SOURCES =
+libthunar_a_LIBADD = 							\
 	$(filter %.o, $(thunar_SOURCES:%.c=thunar-%.o))			\
 	$(wildcard $(top_builddir)/thunarx/.libs/*.o)
 
@@ -350,4 +350,4 @@ EXTRA_DIST =								\
 	thunar.gresource.xml					\
 	$(desktop_in_files)
 
-dist-hook: thunar.a
+dist-hook: libthunar.a