The first PR adds helper macros, AC_CACHE_CHECK() [1] and AS_VAR_IF() [2]. It also unified internal variables to use format "ac_cv_func_$funcname", "ac_cv_func_lib_$library_$funcname", or "ac_cv_header_$headername_h". "ac_cv" stands for autoconf cached value.

AC_CACHE_CHECK() replaces AC_MSG_CHECKING() and AC_MSG_RESULT(). The syntax is AC_CACHE_CHECK([text], [cache variable], [body]) where body is only excecuted when the cache variable is not set. The body has to set the cache variable to yes or no. Any output and AC_DEFINE must occur outside the body.

AS_VAR_IF() is a nicer way to write if test $variable = value; then; fi.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Caching-Results.html#Caching-Results
[2] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Polymorphic-Variables.html