Enable MySQL socket search during the build

See https://github.com/php/php-src/blob/cf3b852109a88a11370d0207cd3b72a53b6a64c3/ext/mysqli/config.m4#L112

Author: kasparsd

build.sh

@@ -34,6 +34,9 @@ git fetch --tags --prune
 git checkout tags/php-7.2.8
 ./buildconf --force
 
+# Enable MySQL socket search during the build.
+export PHP_MYSQL_SOCK=yes
+
 CONFIGURE_STRING="--prefix=/usr/local/php7 \
                   --enable-huge-code-pages \
                   --with-config-file-scan-dir=/usr/local/php7/etc/conf.d \