exporting system verilog functions and task to C using DPI

Most likely you are using an unsupported gcc version. Check the compatibility of gcc versions with your version of modelsim.

If you have ModelSim 10.1 or later, you can put your C file on the vlog command line and it will compile the C file for you

vlog -dpiheader dpiheader.h helloc.sv helloc.c

(Do not use the -sv switch if your SV files already have a .sv extension, otherwise all files including .v will be treated as SystemVerilog)