Add PHP build configuration variables to help info

This marks some environment variables as precious and they show in the
`./configure --help` output. These variables can be used like this:

    ./configure PHP_BUILD_SYSTEM="..." PHP_BUILD_PROVIDER="..."

when customizing the build system info on some systems and are then
displayed in the `phpinfo()` output.

Author: petk

configure.ac

@@ -1495,18 +1495,23 @@ EXTRA_LDFLAGS="$EXTRA_LDFLAGS $PHP_LDFLAGS"
 EXTRA_LDFLAGS_PROGRAM="$EXTRA_LDFLAGS_PROGRAM $PHP_LDFLAGS"
 
 UNAME=`uname -a | xargs`
-PHP_UNAME=${PHP_UNAME:-$UNAME}
+AC_ARG_VAR([PHP_UNAME],[Overrides system information (uname -a output)])dnl
+AS_IF([test -z "$PHP_UNAME"],[PHP_UNAME=[$]UNAME])
 AC_DEFINE_UNQUOTED(PHP_UNAME,"$PHP_UNAME",[uname -a output])
 PHP_OS=`uname | xargs`
 AC_DEFINE_UNQUOTED(PHP_OS,"$PHP_OS",[uname output])
-PHP_BUILD_SYSTEM=${PHP_BUILD_SYSTEM:-$PHP_UNAME}
+AC_ARG_VAR([PHP_BUILD_SYSTEM],[PHP builder name output])dnl
+AS_IF([test -z "$PHP_BUILD_SYSTEM"],[PHP_BUILD_SYSTEM=[$]PHP_UNAME])
 AC_DEFINE_UNQUOTED(PHP_BUILD_SYSTEM,"$PHP_BUILD_SYSTEM",[builder uname output])
+AC_ARG_VAR([PHP_BUILD_PROVIDER],[PHP build provider])dnl
 if test -n "${PHP_BUILD_PROVIDER}"; then
   AC_DEFINE_UNQUOTED(PHP_BUILD_PROVIDER,"$PHP_BUILD_PROVIDER",[build provider])
 fi
+AC_ARG_VAR([PHP_BUILD_COMPILER],[Compiler used for building PHP])dnl
 if test -n "${PHP_BUILD_COMPILER}"; then
   AC_DEFINE_UNQUOTED(PHP_BUILD_COMPILER,"$PHP_BUILD_COMPILER",[used compiler for build])
 fi
+AC_ARG_VAR([PHP_BUILD_ARCH],[PHP build architecture])dnl
 if test -n "${PHP_BUILD_ARCH}"; then
   AC_DEFINE_UNQUOTED(PHP_BUILD_ARCH,"$PHP_BUILD_ARCH",[build architecture])
 fi