configure.ac: Make it possible to build on Android Termux

The recent Termux's clang predefines `__ANDROID_API__` as a fixed number
24. However, when it is lower to the current device version, some
functions that "configure" detemines available are not declared
correctly in the header files.

This change forces to overwrite the predefined `__ANDROID_API__` macro
with the current device version.

ref: termux/termux-packages#6176

configure.ac

@@ -458,17 +458,10 @@ AC_TRY_COMPILE([
 	AC_MSG_RESULT(yes)
 	target_os=${target_os}-android
 	AC_MSG_CHECKING([for Android API version])
-	AC_TRY_COMPILE([
-		@%:@ifndef __ANDROID_API__
-		@%:@error android
-		@%:@endif
-	], [],
-	[AC_MSG_RESULT(defined by the compiler)],
-	[
-		rb_android_api=`getprop ro.build.version.sdk`
-		AC_MSG_RESULT($rb_android_api)
-		RUBY_APPEND_OPTIONS(CPPFLAGS, -D__ANDROID_API__=$rb_android_api)
-	])
+        # hacky workaround: https://github.com/termux/termux-packages/issues/6176
+        rb_android_api=`getprop ro.build.version.sdk`
+        AC_MSG_RESULT($rb_android_api)
+        RUBY_APPEND_OPTIONS(CPPFLAGS, -D__ANDROID_API__=$rb_android_api -Wno-macro-redefined)
 ])
 
 AC_SUBST(RM, ['rm -f'])