Normally, libraries are created using a library manager such as ar. Given a list of objects, the library manager will insert the objects into a named library to be used in subsequent link steps.
xiar cru user.a a.o b.o
The above command creates a library named user.a that contains the a.o and b.o objects.
If, however, the objects have been created using -ipo -c, then the objects will not contain a valid object but only the intermediate representation (IR) for that object file. For example:
ifort -ipo -c a.f b.f
will produce a.o and b.o that only contains IR to be used in a link time compilation. The library manager will not allow these to be inserted in a library.
In this case, you must use the Intel library driver xild -ar. This program will invoke the compiler on the IR saved in the object file and generate a valid object that can be inserted in a library.
xild -lib cru user.a a.o b.o
See Creating a Multifile IPO Executable Using xild.