Fix warnings for functions introduced in Android API level 18.
* m4/getdelim.m4 (gl_FUNC_GETDELIM): Test for getdelim using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS_ONCE.
* m4/getline.m4 (gl_FUNC_GETLINE): Test for getline using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNC.
* m4/log2.m4 (gl_FUNC_LOG2): Test for log2 using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/log2f.m4 (gl_FUNC_LOG2F): Test for log2f using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.
* m4/log2l.m4 (gl_FUNC_LOG2L): Test for log2l using
gl_CHECK_FUNCS_ANDROID instead of AC_CHECK_FUNCS.