Add note for the compatibility.
* src/gpg-error-config-main.sh (--modversion): New. Add comments.
Not supported by pkg-config:
No such options: --prefix --exec-prefix --host Conditional flag which change the output: --mt Different semantics: --version which print the tool version
We keep the backward compatibility for older gpg-error-config
for --mt and other options for a while.
New usage (pkg-config compatible way) is encouraged.