build: fix library installation on cygwin

This installs libraries using the proper names and locations,
generates an import lib for the DLL, and drops no longer needed
linker flags.

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -2600,7 +2600,10 @@ case $target_os in
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(FULLNAME)-$(LIBMAJOR)$(SLIBSUF)'
-        SHFLAGS='-shared -Wl,--enable-auto-image-base'
+        SLIB_INSTALL_NAME='$(SLIBNAME_WITH_MAJOR)'
+        SLIB_INSTALL_LINKS=
+        SLIB_INSTALL_EXTRA_LIB='lib$(FULLNAME).dll.a'
+        SHFLAGS='-shared -Wl,--out-implib,$(SUBDIR)lib$(FULLNAME).dll.a'
         objformat="win32"
         enable dos_paths
         check_cflags -fno-common