mpc: also rename the attribute to libmpc

Attrnames and package names should be as close as possible to avoid confusion.
I took care not to confuse the two mpc things during the mass-replace,
so hopefully I suceeded (tarball still builds).

pkgs/development/compilers/llvm/3.4/dragonegg.nix

@@ -1,4 +1,4 @@
-{stdenv, fetch, fetchpatch, llvm, gmp, mpfr, mpc, ncurses, zlib, version}:
+{stdenv, fetch, fetchpatch, llvm, gmp, mpfr, libmpc, ncurses, zlib, version}:
 
 stdenv.mkDerivation rec {
   name = "dragonegg-${version}";
@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
   # The gcc the plugin will be built for (the same used building dragonegg)
   GCC = "gcc";
 
-  buildInputs = [ llvm gmp mpfr mpc ncurses zlib ];
+  buildInputs = [ llvm gmp mpfr libmpc ncurses zlib ];
 
   installPhase = ''
     mkdir -p $out/lib $out/share/doc/${name}