configure.ac: Rename AM_LDFLAGS to LTLDFLAGS and actually use them

The new variable name tries to clarify that libtool is being used.
Linker flags must thus always be specified with -Wl.

Factor out the libtool flag -no-undefined from host specific cases.
The flag is required to build a Windows DLL, but is correct also for
the other supported systems.

Also, start actually using LTLDFLAGS in libusb/Makefile.am, so that
libtool will see the options set by configure.
diff --git a/configure.ac b/configure.ac
index f810473..95623f2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -15,9 +15,7 @@
 lt_current="1"
 lt_revision="0"
 lt_age="1"
-AC_SUBST(lt_current)
-AC_SUBST(lt_revision)
-AC_SUBST(lt_age)
+LTLDFLAGS="-version-info ${lt_current}:${lt_revision}:${lt_age}"
 
 AM_INIT_AUTOMAKE
 AM_MAINTAINER_MODE
@@ -34,6 +32,8 @@
 AM_PROG_CC_C_O
 AC_DEFINE([_GNU_SOURCE], 1, [Use GNU extensions])
 
+LTLDFLAGS="${LTLDFLAGS} -no-undefined"
+
 AC_MSG_CHECKING([operating system])
 case $host in
 *-linux*)
@@ -45,7 +45,6 @@
 	threads="posix"
 	THREAD_CFLAGS="-pthread"
 	PC_LIBS_PRIVATE="${PC_LIBS_PRIVATE} -pthread"
-	AM_LDFLAGS="-Wl,--no-undefined"
 	AC_CHECK_HEADERS([poll.h])
 	AC_DEFINE([POLL_NFDS_TYPE],[nfds_t],[type of second poll() argument])
 	;;
@@ -56,7 +55,7 @@
 	backend="darwin"
 	threads="posix"
 	PC_LIBS_PRIVATE="-Wl,-framework,IOKit -Wl,-framework,CoreFoundation"
-	AM_LDFLAGS="-Wl,-prebind -Wl,--no-undefined"
+	LTLDFLAGS="${LTLDFLAGS} -Wl,-prebind"
 	AC_CHECK_HEADERS([poll.h])
 	AC_CHECK_TYPE([nfds_t],
 		[AC_DEFINE([POLL_NFDS_TYPE],[nfds_t],[type of second poll() argument])],
@@ -69,7 +68,7 @@
 	AC_MSG_RESULT([Windows])
 	backend="windows"
 	PC_LIBS_PRIVATE="-lsetupapi -lole32 -ladvapi32"
-	AM_LDFLAGS="-no-undefined -avoid-version --add-stdcall-alias"
+	LTLDFLAGS="${LTLDFLAGS} -avoid-version -Wl,--add-stdcall-alias"
 	AC_CHECK_TOOL(RC, windres, no)
 	AC_DEFINE([POLL_NFDS_TYPE],[unsigned int],[type of second poll() argument])
 	;;
@@ -80,7 +79,7 @@
 	backend="windows"
 	threads="posix"
 	PC_LIBS_PRIVATE="-lsetupapi -lole32 -ladvapi32"
-	AM_LDFLAGS="-no-undefined -avoid-version --add-stdcall-alias"
+	LTLDFLAGS="${LTLDFLAGS} -avoid-version -Wl,--add-stdcall-alias"
 	AC_CHECK_TOOL(RC, windres, no)
 	AC_DEFINE([POLL_NFDS_TYPE],[unsigned int],[type of second poll() argument])
 	;;
@@ -188,7 +187,7 @@
 
 AC_SUBST(VISIBILITY_CFLAGS)
 AC_SUBST(AM_CFLAGS)
-AC_SUBST(AM_LDFLAGS)
+AC_SUBST(LTLDFLAGS)
 
 AC_CONFIG_FILES([libusb-1.0.pc])
 AC_CONFIG_FILES([Makefile])