This CL makes the following substitutions.

* assert.h -> cassert
* math.h -> cmath
* stdarg.h -> cstdarg
* stddef.h -> cstddef
* stdint.h -> cstdint
* stdio.h -> cstdio
* stdlib.h -> cstdlib
* string.h -> cstring

PiperOrigin-RevId: 309080151

External linkage is the default for function declarations in C++.

This also fixes ClangTidy errors generated by removing the "extern"
keyword as described above.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188730416

git-svn-id: https://leveldb.googlecode.com/svn/trunk@23 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@21 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@20 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@2 62dab493-f737-651d-591e-8d6aee1b9529