~ CI: install libfl-dev on Linux for FlexLexer.h

Newer Ubuntu ships FlexLexer.h in libfl-dev rather than flex, so
cmake's FindFLEX left FLEX_INCLUDE_DIR NOTFOUND on the runner.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Eric Prud'hommeaux

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
         if: runner.os == 'Linux'
         run: |
           sudo apt-get update
-          sudo apt-get install -y cmake bison flex libexpat1-dev uuid-dev \
+          sudo apt-get install -y cmake bison flex libfl-dev libexpat1-dev uuid-dev \
             libboost-dev libboost-regex-dev libboost-thread-dev \
             libboost-filesystem-dev libboost-date-time-dev \
             libboost-program-options-dev libboost-iostreams-dev \

README

@@ -21,7 +21,7 @@ CI (.github/workflows/ci.yml) runs the same steps on Linux and macOS.
 you'll need these packages:
 
 - cmake
-- flex (>= 2.6)
+- flex (>= 2.6) and libfl-dev (FlexLexer.h)
 - bison (>= 3.2)
 - uuid-dev
 - libexpat1-dev (or libxml2-dev with -DSWOBJ_XML_PARSER=LIBXML2)