understanding compilation commands C and configuring CodeBlocks - c

Question

I'm learning C in college. And I wanted to try new things, so I decided to use the allegro Game library to create some stuff. I followed the tutorial, and everything went fine. I wrote a piece of code in the text editor and executed the commands provided by the tutorial and it compiled and run. (I'm on linux btw).
These are the commands:
gcc hello.c -o hello $(pkg-config allegro-5 allegro_font-5 --libs --cflags)
./hello
So, I understand that gcc is calling the gcc compiler and hello.c is the name of the source code file and -o hello specifies the name of the compiled file. but the next part is ambiguous and beyond my knowledge:
$(pkg-config allegro-5 allegro_font-5 --libs --cflags)
So can anyone explain what it means (it has something to do with linking). and if I remove that part the compiler returns these error codes:
/usr/bin/ld: /tmp/ccIQsEis.o: in function `main':
hello.c:(.text+0x2f): undefined reference to `al_install_system'
/usr/bin/ld: hello.c:(.text+0x34): undefined reference to `al_install_keyboard'
/usr/bin/ld: hello.c:(.text+0x45): undefined reference to `al_create_timer'
/usr/bin/ld: hello.c:(.text+0x4e): undefined reference to `al_create_event_queue'
/usr/bin/ld: hello.c:(.text+0x61): undefined reference to `al_create_display'
/usr/bin/ld: hello.c:(.text+0x6a): undefined reference to `al_create_builtin_font'
/usr/bin/ld: hello.c:(.text+0x73): undefined reference to `al_get_keyboard_event_source'
/usr/bin/ld: hello.c:(.text+0x85): undefined reference to `al_register_event_source'
/usr/bin/ld: hello.c:(.text+0x91): undefined reference to `al_get_display_event_source'
/usr/bin/ld: hello.c:(.text+0xa3): undefined reference to `al_register_event_source'
/usr/bin/ld: hello.c:(.text+0xaf): undefined reference to `al_get_timer_event_source'
/usr/bin/ld: hello.c:(.text+0xc1): undefined reference to `al_register_event_source'
/usr/bin/ld: hello.c:(.text+0xd4): undefined reference to `al_start_timer'
/usr/bin/ld: hello.c:(.text+0xe7): undefined reference to `al_wait_for_event'
/usr/bin/ld: hello.c:(.text+0x125): undefined reference to `al_is_event_queue_empty'
/usr/bin/ld: hello.c:(.text+0x13d): undefined reference to `al_map_rgb'
/usr/bin/ld: hello.c:(.text+0x16a): undefined reference to `al_clear_to_color'
/usr/bin/ld: hello.c:(.text+0x17e): undefined reference to `al_map_rgb'
/usr/bin/ld: hello.c:(.text+0x1ca): undefined reference to `al_draw_text'
/usr/bin/ld: hello.c:(.text+0x1cf): undefined reference to `al_flip_display'
/usr/bin/ld: hello.c:(.text+0x1e7): undefined reference to `al_destroy_font'
/usr/bin/ld: hello.c:(.text+0x1f3): undefined reference to `al_destroy_display'
/usr/bin/ld: hello.c:(.text+0x1ff): undefined reference to `al_destroy_timer'
/usr/bin/ld: hello.c:(.text+0x20b): undefined reference to `al_destroy_event_queue'
collect2: error: ld returned 1 exit status
And for the second part of the question, I tried to compile this code in code:blocks but it returns the same errors given by the compiler without the term: $(pkg-config allegro-5 allegro_font-5 --libs --cflags)
So, what configuration should I change in codeblocks so that it compiles well code using allegro libraries.
PS. I didn't include the source code intentionally as it adds no useful information to the question, only clutters it.

Answer 1

This is a Bash command-syntax feature which executes another command and then provides its output as a string. This idiom is being used here to programmatically provide command-line parameters to the gcc command. The pkg-config command is being executed, and it returns a string which becomes part of – is textually included in – the command-string for invoking gcc. (Which of course neither knows nor cares where its command-line parameters "come from.")
(Clever, huh?)
For example, try this:
echo $(ls)
You will be rewarded with a list of all the files in the current directory, concatenated as a string.

Answer 2

To elaborate on the answer by #Mike Robinson,
$(pkg-config allegro-5 allegro_font-5 --libs --cflags) will execute the command pkg-config and substitute the result. pkg-config is build utility that helps when compiling C programs. It's not specific to allegro, it is used by many C libraries.
In this particular case, pkg-config is called with the parameters allegro-5 allegro_font-5 --libs --cflags, which asks pkg-config "what are the compiler options (--cflags) and libraries (--libs) I need when compiling and linking with allegro?" Allegro consists of several modules, in this case you chose the main module 'allegro' and the font module 'allegro_font'. In both cases you're referring to major version 5 of allegro.
On your system, the result of the command is -I/usr/include/x86_64-linux-gnu -lallegro_font -lallegro. The meaning of these options is as follows:
-I/usr/include/x86_64-linux-gnu tells the compiler to add the path /usr/include/x86_64-linux-gnu to the header search path (because the allegro headers are located there)
-lallegro_font and '-lallegro` tells the linker to add the allegro_font and allegro libraries respectively. That also explains why you get all those undefined reference errors when you remove this - you are no longer telling the linker to add the allegro library, so it can't find all those functions anymore.