Fix Wstrict-prototypes warnings

[petk]

This changes the C function prototypes to use void where needed. When building with CFLAGS=-Werror=strict-prototypes or similar, the libtool's tests can cause false reports and errors in the log files:

error: function declaration isn't a prototype

Related to https://github.com/php/php-src/pull/13732

Edit: patch sent also to the libtool-patches mailing list.