Compilation issues with ICU 68

Hello, and first of all, thanks for the work done so far.

#

There seems to be no possibility to build using asdf-php when ICU is v68 or above. Indeed, as stated here http://site.icu-project.org/download/68, the v68 ICU library doesn't define `TRUE` and `FALSE`, which results in the following issue (regardless of minor version installed, the 7.{0-2} cannot build), example for 7.1.33) :

```
/tmp/tmp.JS3ZQ5bJJH/php-src-php-7.1.33/ext/intl/timezone/timezone_class.cpp: In function ‘HashTable* TimeZone_get_debug_info(zval*, int*)’:
/tmp/tmp.JS3ZQ5bJJH/php-src-php-7.1.33/ext/intl/timezone/timezone_class.cpp:324:28: error: ‘FALSE’ was not declared in this scope
324 | tz->getOffset(now, FALSE, rawOffset, dstOffset, uec);
| ^~~~~
make: *** [Makefile:1244: ext/intl/timezone/timezone_class.lo] Error 1
make: *** Waiting for unfinished jobs....
/tmp/tmp.JS3ZQ5bJJH/php-src-php-7.1.33/ext/intl/timezone/timezone_methods.cpp: In function ‘void zif_intltz_from_date_time_zone(zend_execute_data*, zval*)’:
/tmp/tmp.JS3ZQ5bJJH/php-src-php-7.1.33/ext/intl/timezone/timezone_methods.cpp:94:64: error: ‘FALSE’ was not declared in this scope
94 | tz = timezone_convert_datetimezone(tzobj->type, tzobj, FALSE, NULL,
| ^~~~~
make: *** [Makefile:1246: ext/intl/timezone/timezone_methods.lo] Error 1
```
From what I could gather, the a "temporary" way to fix this would be to define the `U_DEFINE_FALSE_AND_TRUE` to `1` for `gcc` not to error at compile time, but I don't really know where or how it would fit with the rest of the existing code (in asdf-php), or if it even should be considered.

Any ideas / suggestions ?

Start by reproducing the PHP 7.1.33 build with ICU 68 or newer and inspect the reported ext/intl/timezone/timezone_class.cpp and timezone_methods.cpp failures. Determine where asdf-php configures the PHP build and verify completion by building the affected PHP 7.x versions successfully with ICU 68+.

Written by the indexing model from the issue text.