Checking 4 spaces indent in C files in CI

[junaruga]

Seeing the PR https://github.com/ruby/openssl/pull/973 (thanks for the PR!), I am considering if we can check if the indent is 4 spaces without tabs in CI. Right now I am seeing the following tools.

• clang-format: https://clang.llvm.org/docs/ClangFormat.html
• GNU indent: https://www.gnu.org/software/indent/

If we only check if C files don't include tabs. We may just apply the following kind of script.

$ cat check_no_tab.sh
#!/bin/bash

set -eu

if grep -rPn '\t' ext/openssl/; then
    echo "FAIL"
    exit 1
fi

echo "OK"
exit 0

OK case:

$ ./check_no_tab.sh
OK

Error case:

$ ./check_no_tab.sh
ext/openssl/ossl_ssl.c:3302:	DefIVarID(hostname);
FAIL

What do you think?