new entries

library/_toc.md

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 # Popular required libraries
 
+- [Allegro](allegro.md)
 - [Boost](boost.md)
 - [LZMA SDK](lzma.md)
 - [Simple DirectMedia Layer (SDL 2)](sdl_2.md)

library/allegro.md

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+# Allegro
+
+_Cross-platform library mainly aimed at video game and multimedia programming._
+
+- Home: http://liballeg.org/
+- Wikipedia: https://en.wikipedia.org/wiki/Allegro_(software)
+- Download: http://liballeg.org/download.html
+- State: mature
+- Code: primary repository (type if not git), other repositories (type if not git)
+- Language(s): C
+- License: zlib
+
+## Building
+
+Uses CMake
+