build: make mingw-std-threads a submodule

.gitmodules

@@ -14,3 +14,6 @@
 [submodule "src/3rdparty/pegtl"]
 	path = src/3rdparty/pegtl
 	url = ../../lolengine/pegtl.git
+[submodule "src/3rdparty/mingw-std-threads"]
+	path = src/3rdparty/mingw-std-threads
+	url = ../../lolengine/mingw-std-threads.git

build/autotools/m4/lol-conf.m4

@@ -72,22 +72,13 @@ AC_TRY_LINK([], [],
 AC_LANG_POP(C++)
 
 
-dnl  Use mingw threads if necessary
-if true; then
-  LOL_CFLAGS="$LOL_CFLAGS -I\$(lol_srcdir)/external/mingw-std-threads-0.0.ac5c9e3b/include/"
-fi
-
-
-dnl  Use pegtl? Yes, always
-if true; then
-  LOL_CFLAGS="$LOL_CFLAGS -I\$(lol_srcdir)/src/3rdparty/pegtl"
-fi
-
+dnl
+dnl  Activate some header-only libraries
+dnl
 
-dnl  Use Imgui? Yes, always
-if true; then
-  LOL_CFLAGS="$LOL_CFLAGS -I\$(lol_srcdir)/src/3rdparty/imgui"
-fi
+LOL_CFLAGS="$LOL_CFLAGS -I\$(lol_srcdir)/src/3rdparty/mingw-std-threads"
+LOL_CFLAGS="$LOL_CFLAGS -I\$(lol_srcdir)/src/3rdparty/pegtl"
+LOL_CFLAGS="$LOL_CFLAGS -I\$(lol_srcdir)/src/3rdparty/imgui"
 
 
 dnl  Use NativeClient?

external

@@ -1 +1 @@
-Subproject commit 406ad09301126c1d80c08179973589fe8930162c
+Subproject commit b83ca55097a337ba19f5f8ff66cd1a972f0965c0

src/3rdparty/mingw-std-threads

@@ -0,0 +1 @@
+Subproject commit b7e670d91d33b7ce5836c6255d37e69f17eb3687