macOS: use GCC 10 for building, since GCC 9 is no longer in Homebrew

(cherry picked from commit 64ebb362)

.gitlab-ci.yml

@@ -93,7 +93,7 @@ build_macOS:
     - macOS
   script:
     - autoreconf -si
-    - './configure CXX=g++-9 CXXFLAGS=-static-libgcc'
+    - './configure CXX=g++-10 CXXFLAGS=-static-libgcc'
     - make -j$(nproc)
     - strip src/dynare_m
     - mkdir -p bin