configure: Don't abort if the compiler does not define __STDC__
configure: Don't abort if the compiler does not define __STDC__

This removes a check for $ac_cv_prog_cc_c89 which is set by AC_PROG_CC
if defined(__STDC__) in the preprocessor. (Standard compliant compilers
are supposed to define __STDC__ to 1 but the value is actually not
checked here.)

Unfortunately, MSVC doesn't define it, so configure fails for MSVC.

This check is not very useful in practice. Over 30 years after C89 has
been released, there are no C compilers out there that are not
sufficiently compliant with C89 for the project. The only practically
relevant case was that the check rejected C++ compilers. A different
method to reject C++ compilers will be introduced in a later commit.

bitcoin-core/secp256k1,bitcoin-core/secp256k1