gnu: make-libiberty: Build with -fPIC.

Since iberty is a static library, linking a PIC-enabled target to it
requires it to also have been built with PIC.  (The target is Dyninst
in this case.)

This `-fPIC' flag is appended to the default CFLAGS of `-O2 -g'.

* gnu/packages/gcc.scm (make-libiberty)[arguments]:
  Adjust make-flags to add -fPIC to CFLAGS.

Change-Id: I155045d05f4434cb68be933a95b7bc9fdec98818
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/gcc.scm

@@ -1065,6 +1065,7 @@ using compilers other than GCC."
     (name "libiberty")
     (arguments
      `(#:out-of-source? #t
+       #:make-flags '("CFLAGS=-O2 -g -fPIC")
        #:phases
        (modify-phases %standard-phases
          (add-before 'configure 'chdir