configure.in: Rename to configure.ac to fix an 'aclocal' warning

Also rename references in related files.

CMakeLists.txt

@@ -771,7 +771,7 @@ if(SDL_POWER)
   file(GLOB POWER_SOURCES ${SDL2_SOURCE_DIR}/src/power/*.c)
   set(SOURCE_FILES ${SOURCE_FILES} ${POWER_SOURCES})
 endif()
-# TODO: in configure.in, the test for LOADSO and SDL_DLOPEN is a bit weird:
+# TODO: in configure.ac, the test for LOADSO and SDL_DLOPEN is a bit weird:
 # if LOADSO is not wanted, SDL_LOADSO_DISABLED is set
 # If however on Unix or APPLE dlopen() is detected via CheckDLOPEN(),
 # SDL_LOADSO_DISABLED will not be set, regardless of the LOADSO settings
@@ -1301,7 +1301,7 @@ elseif(WINDOWS)
   # Libraries for Win32 native and MinGW
   list(APPEND EXTRA_LIBS user32 gdi32 winmm imm32 ole32 oleaut32 version uuid advapi32 shell32)
 
-  # TODO: in configure.in the check for timers is set on
+  # TODO: in configure.ac the check for timers is set on
   # cygwin | mingw32* - does this include mingw32CE?
   if(SDL_TIMERS)
     set(SDL_TIMER_WINDOWS 1)
@@ -1569,7 +1569,7 @@ if(VIDEO_VULKAN)
 endif()
 
 # Dummies
-# configure.in does it differently:
+# configure.ac does it differently:
 # if not have X
 #   if enable_X {  SDL_X_DISABLED = 1 }
 #   [add dummy sources]

Makefile.in

@@ -44,7 +44,7 @@ SDLTEST_OBJECTS = @SDLTEST_OBJECTS@
 
 WAYLAND_SCANNER = @WAYLAND_SCANNER@
 
-SRC_DIST = *.txt acinclude Android.mk autogen.sh android-project build-scripts cmake cmake_uninstall.cmake.in configure configure.in debian docs include Makefile.* sdl2-config.cmake.in sdl2-config.in sdl2.m4 sdl2.pc.in SDL2.spec.in SDL2Config.cmake src test VisualC.html VisualC VisualC-WinRT Xcode Xcode-iOS wayland-protocols
+SRC_DIST = *.txt acinclude Android.mk autogen.sh android-project build-scripts cmake cmake_uninstall.cmake.in configure configure.ac debian docs include Makefile.* sdl2-config.cmake.in sdl2-config.in sdl2.m4 sdl2.pc.in SDL2.spec.in SDL2Config.cmake src test VisualC.html VisualC VisualC-WinRT Xcode Xcode-iOS wayland-protocols
 GEN_DIST = SDL2.spec
 
 ifneq ($V,1)
@@ -127,7 +127,7 @@ LT_LDFLAGS  = -no-undefined -rpath $(libdir) -release $(LT_RELEASE) -version-inf
 
 all: $(srcdir)/configure Makefile $(objects)/$(TARGET) $(objects)/$(SDLMAIN_TARGET) $(objects)/$(SDLTEST_TARGET)
 
-$(srcdir)/configure: $(srcdir)/configure.in
+$(srcdir)/configure: $(srcdir)/configure.ac
 	@echo "Warning, configure is out of date, please re-run autogen.sh"
 
 Makefile: $(srcdir)/Makefile.in

configure.ac

@@ -3913,7 +3913,7 @@ AS_HELP_STRING([--enable-render-d3d], [enable the Direct3D render driver [[defau
         ;;
     *)
         AC_MSG_ERROR([
-*** Unsupported host:  Please add to configure.in
+*** Unsupported host:  Please add to configure.ac
         ])
         ;;
 esac