This replaces the AC_MSG_ERROR with AC_MSG_FAILURE, where appropriate.
The AC_MSG_ERROR outputs given message and exits the configure step. The
AC_MSG_FAILURE does the same but also automatically outputs additional
message "See 'config.log' for more details." which might help directing
the user where to look further.
The AC_MSG_ERROR is used for errors where current test step isn't logged
in the config.log and wouldn't make sense, and AC_MSG_FAILURE is mostly
used in cases of library checks, compilation tests, headers checked with
AC_CHECK_HEADER* and similar tests that are also logged in the
config.log.
AC_MSG_ERROR([Sanity check failed.]) output:
```
configure: error: Sanity check failed.
```
AC_MSG_FAILURE([Sanity check failed.]) output:
```
configure: error: in '/path/to/php-src':
configure: error: Sanity check failed.
See 'config.log' for more details
```
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