libsecp256k1.pc returns incorrect ordering of libraries for static linking #569

When gmp is detected and the intent is static linking, the invocation of pkg-config --libs --static libsecp256k1 returns -L/mnt/alpha/prefix/static/lib -lgmp -lsecp256k1. Note that the order of -l arguments is reversed to that needed for successful linking. While autotools use of libtool reorders this, other users of pkg-config run the risk of linker errors utilizing the current output.