configure: conditionally enable pedantic-errors

Enable pedantic-errors for GCC >= 5 with --enable-werror. Before GCC 5,
pedantic-errors was synonymous to -Werror=pedantic [0], which is still
the case for clang [1]. With GCC 5, it became complementary [2].

Also fix a resulting error in acinclude.m4 as main's return type was
missing, which is illegal in C99.

[0] https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Warning-Options.html
[1] https://clang.llvm.org/docs/UsersManual.html#options-to-control-error-and-warning-messages
[2] https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Warning-Options.html

Closes https://github.com/curl/curl/pull/2747