String.h Header File

I am running RHEL workstation 7.3 and I installed Development Tools to get access to gcc and g++ compiler. As part of development tools there was also this file string.h that was installed in my /usr/include/ folder that wasnt there before. Now the problem is that I am unable to use this header file in any of my c programs that I want to compile and execute. When I try to compile my c program I get an error

/usr/include/string.h:409:14: error: expected identifier or ‘(’ before string constant
extern char *strerror (int __errnum) __THROW

Does anybody know the source of this file ?