[问题]flex是什么？

昨天装一个软件的时候./configure后遇到下面的问题，错误代码见最后。

网上查到有什么“FLEX 2.01 全套下载：客户端:
Flex Builder 2 with Charting (集成 Flex Builder 2 + Flex 2 SDK + Flex 2 Charting, 大小:168.78MB)

Flex Builder 2 Eclipse plug-in (集成 Flex Builder 2 + Flex 2 SDK + Flex 2 Charting 作为 Eclipse 的插件, 大小:168.78MB)

Flex Builder 2.01 Updater (Flex Builder 重要更新 2.01, 大小:144MB)

服务器端:

Flex Data Services 2 Express (完全免费, 但只能用于单核心CPU电脑上, 大小:93.73MB)

组件单独下载:
Flex 2 SDK (包含 Flex 2 所有组件的类库 和 Flex编译器, 大小:38.07MB)

Flex Charting 2 (Flex 2 图表组件, 大小:17.04MB)

Flex Automation Package (Flex 测试包, 仅提供给 Flex Data Services 2 企业版用户使用, 大小:104.24MB)”

Ⅰ这么多而且又这么大，我该下载哪个啊？
Ⅱ我就只缺这个flex吗？

十分感谢！

./configure后的结果：
checking build system type... i686-pc-linux-gnu
checking host system type... i686-pc-linux-gnu
checking whether make sets $(MAKE)... yes
checking for gcc... gcc
checking for C compiler default output file name... a.out
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables...
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking for g++... g++
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
checking for cpp... cpp
checking for the directory containing the Wine tools... $(TOPOBJDIR)
checking how to run the C preprocessor... gcc -E
checking for X... no
checking for flex... no
configure: error: no suitable flex found. Please install the 'flex' package.