Accounting for SHELL in ./configure

Dear Freetype developers,

I was trying to build FreeType from source and noticed that the 
'./configure' script does not account for the 'SHELL' environment and 
will always use '/bin/sh'.

Looking at the source of the './configure' script, I was able to fix the 
problem by manually adding a 'SHELL=$SHELL' in line 135 of the 
'./configure' script:

https://gitlab.freedesktop.org/freetype/freetype/-/blob/master/configure?ref_type=heads#L135

Accounting for the user's given SHELL is common in many programs when 
building from source, so it would be good if you could account for it in 
future versions of FreeType also.

Cheers,
Mohammad