MAINT: Include eigen include path

Author: rmcar17

build_tools/before_all_mac.sh

@@ -6,7 +6,7 @@ fi
 brew install llvm eigen@3 boost libomp make
 
 export LDFLAGS="-L$(brew --prefix libomp)/lib"
-export CPPFLAGS="-I$(brew --prefix libomp)/include"
+export CPPFLAGS="-I$(brew --prefix libomp)/include -I$(brew --prefix eigen@3)/include"
 export CXXFLAGS="-I$(brew --prefix libomp)/include"
 
 bash build_tools/build_iqtree.sh