configure.ac, search for BLT library

search for three variants
  -lBLT       (for debian and gentoo)
  -lBLT24     (for redhat and centos)
  -lBLT25     (might be usefull for someone else)

The advertised configure incantation is now:
  configure --enable-shared=yes --with-tcl=/usr/lib/tcl8.6
In case you need a special "BLT" you can still use
  LIBS=-lBLTfoobar configure --enable-shared=yes --with-tcl=/usr/lib/tcl8.6
to override the automatic search

configure.ac

@@ -461,7 +461,10 @@ EOF
         AC_MSG_ERROR([Couldn't find BLT])
     fi
     rm -f conftest.tcl
-    
+
+    AC_SEARCH_LIBS([Blt_GetVector], [BLT BLT24 BLT25],,
+        [AC_MSG_ERROR([Couldnt find BLT library.])])
+
     AC_CHECK_LIB([pthread], [pthread_create])
     
     if test -n "$libdir" ; then