Description:
------------
The following commit changed the --with-password-argon2 configuration flag to --with-argon2, and also how to detect the library in the system (i.e. by package name).

https://github.com/php/php-src/commit/bdcef51bcb367323f2eea4808f34a1b3e58bd3d8

However, on Alpine Linux the libargon2-dev package is named argon2-dev, and the new configure script does not detect it even though usr/lib/libargon2.so and 
usr/lib/libargon2.a are present.

Test script:
---------------
./configure --with-argon2

Expected result:
----------------
checking whether to build with Argon2 support... yes
checking for libargon2... yes

Actual result:
--------------
checking whether to build with Argon2 support... yes
checking for libargon2... no
mconfigure: error: Package requirements (libargon2) were not met:

Package 'libargon2', required by 'virtual:world', not found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables ARGON2_CFLAGS
and ARGON2_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.