c++ - error compiling when combining projects -

i have 2 separate working projects on raspberry running raspbian. having problems when joining them , not compiler itself(still learning) don't know how work out.

error (finish_with_error can ignored):

in file included main.c:20:0: /usr/local/include/wiringpi.h:216:21: error: conflicting types ‘bcm2835_delaymicroseconds’ bcm2835.h:912:17: note: previous declaration of ‘bcm2835_delaymicroseconds’ here main.c:200:6: warning: conflicting types ‘finish_with_error’ [enabled default] main.c:124:33: note: previous implicit declaration of ‘finish_with_error’ here 

the command using compile is:

 gcc config.c rfid.c rc522.c main.c -o rc522_reader -lbcm2835 -lwiringpi -lwiringpidev `mysql_config --cflags --libs` 

bcm2835.h(line 912):

extern void bcm2835_delaymicroseconds (uint64_t micros); 

wiringpi.h(line 216):

extern void         delaymicroseconds (unsigned int howlong) ; 

the problem cant see there 2 declarations same way, these libraries prefer not modify avoid malfunctioning on libs, there can tell compiler do?

thanks,