I installed libcurl using :sudo apt-get install libcurl4-openssl-dev. When I try to compile my program I read that we have to pass where libcurl is located to the GCC compiler(although for me its located at /usr/include).
Therefore according to http://ubuntuforums.org/showthread.php?t=1175115 I compiled it using

gcc myfile.c $(pkg-config --libs --cflags libcurl) -o myprogram

But if I only enter pkg-config --cflags libcurl it prints nothing (only a blank space). The program compiles correctly if I add the flag and --libs. I only do not understand why pkg-config --cflags libcurl returns nothing)