build: create a macro to check for, and replace, possibly-inline functions.

mingw (both 32 and 64) provides a number of functions that have no C
linkage, but are only available as static inline. Define a macro that can
check for the function declaration but acts like AC_REPLACE_FUNC.

Use the new macro for asprintf/vasprintf (previously implemented in
configure.ac directly).

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>