C-compilers that no longer work, because they cannot find stdio.h

In the gcc and cc compilers on my laptop, the stdio.h is sought in the directory /usr/include, which in traditional UNIX/LINUX/MacOS installations has contained a boatload of header files (*.h).  Apple has eliminated this directory.  After much searching on message boards, I found this directory on my laptop:

/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include

I copied the directory into the /usr directory

sudo cp -r /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include /usr

in order to write into a root-owned directory.  Once in place, and after refreshing the shell, I avoided the stdio.h error when compiling a sample C subroutine.  I get a number of other warnings, so Im not eager to write more C code in the future, but the command cc -c refft.c produces an object file refft.o that might work in your application.