flex: add page

pages/common/flex.md

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+# flex
+
+> Lexical analyser generator. Based on lex.
+> Given the specification for a lexical analyser, generates C code implementing it.
+
+- Generate an analyser from a flex file:
+
+`lex {{analyser.l}}`
+
+- Specify the output file:
+
+`lex {{analyser.l}} --outfile {{analyser.c}}`
+
+- Compile a C file generated by flex:
+
+`cc {{path/to/lex.yy.c}} --output {{executable}}`