I was working on Allegro 5.0.10 and now I decided to continue my works.
Since I started updating things, Allegro is not working anymore and I can't compile my programs with g++.
First, the compiler was telling me that I need a greater version of allegro (>= 5.0.12) and mine was 5.0.10. The fact is some programs were able to get compiled and working until I started using the native dialog addon.
Second, I updated from github, downloaded the newer version from liballeg.org, downloaded from PPA, but the compiler gives me errors each time anyway.

Errors be like:
(.text+0x20): undefined reference to "main"
(.text+0x20): undefined reference to "al_init"
(.text+0x20): undefined reference to "al_show_native_messagebox"
(.text+0x20): undefined reference to "al_install_..."
etc...
collect2: error: ld returned 1 exit status

What have I done? Can it be fixed?

I tried and it's not working.

Package allegro-5.2 was not found in the pkg-config search path.
Perhaps you should add the directory containing `allegro-5.2.pc'
to the PKG_CONFIG_PATH environment variable
No package 'allegro-5.2' found
Package allegro_acodec-5.2 was not found in the pkg-config search path.
Perhaps you should add the directory containing `allegro_acodec-5.2.pc'
to the PKG_CONFIG_PATH environment variable
No package 'allegro_acodec-5.2' found
Package allegro_audio-5.2 was not found in the pkg-config search path.
Perhaps you should add the directory containing `allegro_audio-5.2.pc'
to the PKG_CONFIG_PATH environment variable
No package 'allegro_audio-5.2' found

...the same with all the packages...

g++: fatal error: no input files
compilation terminated.

With "pkg-config --list-all | grep allegro":
allegro_font-5 allegro_font - Allegro game programming library, font addon
allegro_color-5 allegro_color - Allegro game programming library, colors addon
allegro_image-5 allegro_image - Allegro game programming library, image I/O addon
allegro_memfile-5 allegro_memfile - Allegro game programming library, memory files addon
allegro_ttf-5 allegro_ttf - Allegro game programming library, TrueType fonts addon
allegro_physfs-5 allegro_physfs - Allegro game programming library, PhysicsFS addon
allegro_dialog-5 allegro_dialog - Allegro game programming library, native dialog addon
allegro_acodec-5 allegro_acodec - Allegro game programming library, audio codec addon
allegro_video-5 allegro_video - Allegro game programming library, video player addon
allegro_main-5 allegro_main - Allegro game programming library, magic main addon
allegro-5 allegro - Allegro game programming library
allegro_primitives-5 allegro_primitives - Allegro game programming library, primitives addon
allegro_audio-5 allegro_audio - Allegro game programming library, audio addon

I tried with "g++ -o main.cpp `pkg-config --cflags --libs allegro-5 allegro_acodec-5 allegro_audio-5 allegro_color-5 allegro_dialog-5 allegro_font-5 allegro_image-5 allegro_main-5 allegro_memfile-5 allegro_physfs-5 allegro_primitives-5 allegro_ttf-5`" and the output is:
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: in function "_start":
(.text+0x20): undefined reference to "main"
collect2: error: ld returned 1 exit status

The problem was the name of the program after -o (I forgot it)

Now is working!