How to add multiple header include and library directories to the search path in a single gcc command?

Use multiple -I flags for the include directories and multiple -L flags for the lib directories

You can set the C_INCLUDE_PATH environment variable.

export C_INCLUDE_PATH=.:/some/dir:/some/other/dir

as well as the LIBRARY_PATH environment variable.

On Linux you need to use -I before each directory that you want to add.

Example:

user:/home/my_project$ gcc -g -Wall -I/usr/include/lib_Directory/ -I./include -c ./src/transcod.c

./ means the current directory where you are running the command, in this case my_project;