correct LIBS and LDFLAGS to link correctly

LIBS were in totally wrong place, duh...
Also restore LIBS to the previous state is eclib is not working.

build/pkgs/eclib/spkg-configure.m4

@@ -9,19 +9,19 @@ SAGE_SPKG_CONFIGURE([eclib], [
         dnl header types.h appeared in v20180710
         AC_CHECK_HEADER([eclib/types.h], [
           AC_MSG_CHECKING([whether we can link and run a program using eclib])
-          ECLIB_SAVED_LDFLAGS=$LDFLAGS
-          LDFLAGS="$LDFLAGS -lec"
+          ECLIB_SAVED_LIBS=$LIBS
+          LIBS="$LIBS -lec"
           AC_RUN_IFELSE([
             AC_LANG_PROGRAM([[#include <eclib/version.h>]
                              [#include <eclib/interface.h>]],
                       [[set_bit_precision(42); /* test for versions >= v20190226 */
                         show_version();
                         return 0;]]
-            )], [LIBS="$LIBS -lec"]
-                [AC_MSG_RESULT([yes; use eclib from the system])], [
-            AC_MSG_RESULT([no; install eclib]); sage_spkg_install_eclib=yes
+            )], [AC_MSG_RESULT([yes; use eclib from the system])], [
+            AC_MSG_RESULT([no; install eclib])
+            sage_spkg_install_eclib=yes
+            LIBS=$ECLIB_SAVED_LIBS
           ])
-          LDFLAGS=$ECLIB_SAVED_LDFLAGS
         ], [sage_spkg_install_eclib=yes])
     fi
 ])