CI: on macOS, enforce flex and bison from Homebrew

(cherry picked from commit f5760c33)

.gitlab-ci.yml

@@ -93,7 +93,7 @@ build_macOS:
     - macOS
   script:
     - autoreconf -si
-    - './configure CXX=g++-10 CXXFLAGS=-static-libgcc'
+    - './configure CXX=g++-10 CXXFLAGS=-static-libgcc LEX=/usr/local/opt/flex/bin/flex YACC=/usr/local/opt/bison/bin/bison'
     - make -j$(nproc)
     - strip src/dynare_m
     - mkdir -p bin