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]