Use c++14 for windows build

.github/workflows/ci.yaml

@@ -165,7 +165,12 @@ jobs:
           cd build
           -DBoost_INCLUDE_DIR=${{steps.install-boost.outputs.BOOST_ROOT}}/include\
           -DBoost_LIBRARY_DIRS=${{steps.install-boost.outputs.BOOST_ROOT}}/lib
-          cmake -G "MinGW Makefiles" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_FLAGS=--target=x86_64-pc-windows-gnu -DCMAKE_CXX_FLAGS=--target=x86_64-pc-windows-gnu -DCMAKE_MAKE_PROGRAM=mingw32-make ..
+          cmake -G "MinGW Makefiles" \
+            -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
+            -DCMAKE_C_FLAGS=--target=x86_64-pc-windows-gnu \
+            -DCMAKE_CXX_FLAGS=--target=x86_64-pc-windows-gnu \
+            -DCMAKE_MAKE_PROGRAM=mingw32-make \
+            -DIQTREE_FLAGS="c++14" ..
           make -j
         env:
           BOOST_ROOT: ${{ steps.install-boost.outputs.BOOST_ROOT }}

CMakeLists.txt

@@ -25,6 +25,8 @@
 #
 # nozlib: do not use zlib
 # libcxx
+#
+# c++14: use C++14 standard (default is C++17)
 
 # CMAPLE: Integrate CMAPLE library
 #---------------
@@ -249,7 +251,7 @@ set(CLANG "FALSE") # Clang compiler
 set(ICC "FALSE")   # Intel compiler
 set(VCC "FALSE")   # MS Visual C Compiler, note that it is different from MSVC variable
 set(CLANG_UNDER_VS "FALSE") #Clang compiler, used from inside Visual Studio
-# using C++17 standard
+# using C++17 standard (unless IQTREE_FLAGS specifies c++14, required for compilation using mingw in Github actions)
 # disable AVX for NEON
 if (__ARM_NEON)
     set(IQTREE_FLAGS "${IQTREE_FLAGS} novx")
@@ -258,6 +260,8 @@ elseif (CMAKE_CXX_COMPILER MATCHES "VISUAL STUDIO")
     #it won't recognize the -std=c++17 parameter.
     #Todo: don't hard-code this; figure out some way it can be passed in (though ideally, not the whole shebang).
     include_directories("C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Community\\VC\\Tools\\Llvm\\lib\\clang\\10.0.0\\include")
+elseif(IQTREE_FLAGS MATCHES "c++14")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
 else()
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++17")
 endif()