The only source code that should be placed in a specific location is the Linux kernel source code. It is located in /usr/src/linux.
If a C or C++ compiler is installed, but the complete Linux kernel source code is not installed, then the include files from the kernel source code shall be located in these directories:
/usr/src/linux/include/asm-<arch>
/usr/src/linux/include/linux
<arch> is the name of the system architecture.
Note: /usr/src/linux may be a symbolic link to a kernel source code tree.
It is important that the kernel include files be located in /usr/src/linux and not in /usr/include so there are no problems when system administrators upgrade their kernel version for the first time.