Some files in libcroco will not compile with many C compilers, because
they use the C++ feature of declaring variables anywhere.  The two files
that have this problem are

src/seleng/cr-sel-eng.c
src/layeng/cr-lay-eng.c

To see it, just try building with a 2.9x C compiler.

The fix is simple: just move the declarations to occur before the first
g_return_val_if_fail call.