Ensure consistent compiler warn/error flags

[alex-chew]

In finding #687 we found that, for some compiler configurations, the lines in question yielded compilation errors such as

/.../aws-encryption-sdk-c/source/default_cmm.c:55:31: error: result of comparison of constant 65535 with expression of type 'enum aws_cryptosdk_alg_id' is always false
      [-Werror,-Wtautological-constant-out-of-range-compare]
        if (self->default_alg == DEFAULT_ALG_UNSET) {
            ~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~
1 error generated.

On the other hand, my compiler doesn't error on this; it uses the following warn/error flags:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -x c++ -target x86_64-apple-macos10.15
-fmessage-length=118 -fdiagnostics-show-note-include-stack -fmacro-backtrace-limit=0 -fcolor-diagnostics -Wno-trigraphs -fpascal-strings -O0 -Wno-missing-field-initializers -Wno-missing-prototypes -Wno-return-type -Wno-non-virtual-dtor -Wno-overloaded-virtual -Wno-exit-time-destructors -Wno-missing-braces -Wparentheses -Wswitch -Wno-unused-function -Wno-unused-label -Wno-unused-parameter -Wno-unused-variable -Wunused-value -Wno-empty-body -Wno-uninitialized -Wno-unknown-pragmas -Wno-shadow -Wno-four-char-constants -Wno-conversion -Wno-constant-conversion -Wno-int-conversion -Wno-bool-conversion -Wno-enum-conversion -Wno-float-conversion -Wno-non-literal-null-conversion -Wno-objc-literal-conversion -Wno-shorten-64-to-32 -Wno-newline-eof -Wno-c++11-extensions
-DCMAKE_INTDIR=\"Debug\" -DAWS_ENCRYPTION_SDK_SHARED -DAWS_ENCRYPTION_SDK_TEST_SHARED -DAWS_COMMON_USE_IMPORT_EXPORT -DAWS_SDK_VERSION_MAJOR=1 -DAWS_SDK_VERSION_MINOR=8 -DAWS_SDK_VERSION_PATCH=32 -DAWS_EVENT_STREAM_USE_IMPORT_EXPORT -DAWS_ENCRYPTION_SDK_CPP_SHARED -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk
-fasm-blocks -fstrict-aliasing -Wdeprecated-declarations -Winvalid-offsetof -g -Wno-sign-conversion -Wno-infinite-recursion -Wno-move -Wno-comma -Wno-block-capture-autoreleasing -Wno-strict-prototypes -Wno-range-loop-analysis -Wno-semicolon-before-method-body (snip)

We should determine what causes this discrepancy.

[alex-chew]

I'm running

$ clang --version
Apple clang version 11.0.3 (clang-1103.0.32.62)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin