Fix compilation when OS does not have libusb installed

https://github.com/Hamlib/Hamlib/issues/803

configure.ac

@@ -321,10 +321,11 @@ LIBUSB=""
 AC_ARG_VAR([LIBUSB_CFLAGS], [C compiler flags for libusb, overriding configure defaults])
 AC_ARG_VAR([LIBUSB_LIBS], [linker flags for libusb, overriding configure check (useful for specifying static libusb-1.0.a (see INSTALL))])
 
-AS_IF([test x"${cf_with_libusb}" = "xyes"],
-    [AS_IF([test -z $LIBUSB_LIBS],
+AS_IF([test x"${cf_with_libusb}" = "xyes"], [
+    AS_IF([test -z $LIBUSB_LIBS],
 	[AC_CHECK_LIB([usb-1.0], [libusb_init], [LIBUSB_LIBS="-lusb-1.0"],
 	    [AC_MSG_WARN([libusb_init was not found in libusb-1.0--USB backends will be disabled])])
+        [cf_with_libusb="no"]
 	], [cf_with_libusb="yes"])
 
     AS_IF([test x"${cf_with_libusb}" = "xyes"],

tests/rigtestlibusb.c

@@ -24,12 +24,13 @@
 
 #include <stdio.h>
 #include <string.h>
-#if __has_include("libusb.h")
+#if defined(HAVE_LIBUSB_H)
 #include "libusb.h"
-#else
+#elif defined(HAVE_LIBUSB_1_0_LIBUSB_H)
 #include <libusb-1.0/libusb.h>
 #endif
 
+#if HAVE_LIBUSB
 int verbose = 0;
 
 static void print_endpoint_comp(const struct
@@ -410,3 +411,10 @@ int main(int argc, char *argv[])
     libusb_exit(NULL);
     return r;
 }
+#else
+int main()
+{
+    puts("libusb not installed");
+    return 0;
+}
+#endif