Use syntax that Bourne shell also supports

configure

@@ -3085,7 +3085,7 @@ _ACEOF
 
 fi
 
-   EXTRA_LIBS+=" -lexecinfo" ;
+   EXTRA_LIBS="${EXTRA_LIBS} -lexecinfo" ;
 fi
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for rl_save_prompt in -lreadline" >&5
@@ -3129,7 +3129,7 @@ if test "x$ac_cv_lib_readline_rl_save_prompt" = xyes; then :
 
 $as_echo "#define READLINE_GNU 1" >>confdefs.h
 
-     EXTRA_LIBS+=" -lreadline" ;
+     EXTRA_LIBS="${EXTRA_LIBS} -lreadline" ;
 
 else
 
@@ -3181,7 +3181,7 @@ fi
 
 $as_echo "#define READLINE_EDIT 1" >>confdefs.h
 
-     EXTRA_LIBS+=" -ledit" ;
+     EXTRA_LIBS="${EXTRA_LIBS} -ledit" ;
 
 
 fi
@@ -3225,7 +3225,7 @@ fi
 $as_echo "$ac_cv_lib_rt_clock_gettime" >&6; }
 if test "x$ac_cv_lib_rt_clock_gettime" = xyes; then :
 
-     EXTRA_LIBS+=" -lrt" ;
+     EXTRA_LIBS="${EXTRA_LIBS} -lrt" ;
 
 fi
 
@@ -3285,7 +3285,7 @@ else
   as_fn_error $? "No libconfig found. Try --disable-libconfig" "$LINENO" 5
 fi
 
-    EXTRA_LIBS+=" -lconfig" ;
+    EXTRA_LIBS="${EXTRA_LIBS} -lconfig" ;
   fi
 
 else
@@ -3336,7 +3336,7 @@ else
   as_fn_error $? "No libconfig found. Try --disable-libconfig" "$LINENO" 5
 fi
 
-    EXTRA_LIBS+=" -lconfig" ;
+    EXTRA_LIBS="${EXTRA_LIBS} -lconfig" ;
 
 fi
 

configure.ac

@@ -24,24 +24,24 @@ AC_CHECK_FUNC([backtrace],
 if test "x$BT" = "x" ; then
   AC_MSG_RESULT([no])
   AC_CHECK_LIB([execinfo], [backtrace]) 
-  [ EXTRA_LIBS+=" -lexecinfo" ; ]
+  [ EXTRA_LIBS="${EXTRA_LIBS} -lexecinfo" ; ]
 fi
 
 AC_CHECK_LIB([readline], [rl_save_prompt], 
   [
     AC_DEFINE([READLINE_GNU], [1], [Use gnu libreadline])
-    [ EXTRA_LIBS+=" -lreadline" ; ]
+    [ EXTRA_LIBS="${EXTRA_LIBS} -lreadline" ; ]
   ],
   [
     AC_CHECK_LIB([edit], [rl_set_prompt])
     AC_DEFINE([READLINE_EDIT], [1], [Use libedit])
-    [ EXTRA_LIBS+=" -ledit" ; ]
+    [ EXTRA_LIBS="${EXTRA_LIBS} -ledit" ; ]
   ]
 )
 
 AC_CHECK_LIB([rt], [clock_gettime], 
   [
-    [ EXTRA_LIBS+=" -lrt" ; ]
+    [ EXTRA_LIBS="${EXTRA_LIBS} -lrt" ; ]
   ],
   [
   ]
@@ -55,12 +55,12 @@ AC_ARG_ENABLE(libconfig,[--enable-libconfig/--disable-libconfig],
   else
     AC_MSG_RESULT([enabled])
     AC_CHECK_LIB([config],[config_init],AC_DEFINE(ENABLE_LIBCONFIG,1,[enable libconfig]),AC_MSG_ERROR([No libconfig found. Try --disable-libconfig]))
-    [EXTRA_LIBS+=" -lconfig" ; ]
+    [EXTRA_LIBS="${EXTRA_LIBS} -lconfig" ; ]
   fi 
   ],[
     AC_MSG_RESULT([enabled])
     AC_CHECK_LIB([config],[config_init],AC_DEFINE(ENABLE_LIBCONFIG,1,[enable libconfig]),AC_MSG_ERROR([No libconfig found. Try --disable-libconfig]))
-    [EXTRA_LIBS+=" -lconfig" ; ]
+    [EXTRA_LIBS="${EXTRA_LIBS} -lconfig" ; ]
   ])
     
 # Checks for header files.