More robust parsing and arg validation for build.sh

[DSMitten]

The issue prompting this PR is #1341, where a positional argument following a switch will cause the switch to be swallowed and not evaluated.

This PR changes how commands are parsed so that:

• All positional arguments are parsed before any switches, in a loop
• Any number of positional arguments will be evaluated
• If there is a conflict (such as 'release' and 'debug', or 'arm64' and 'x86_64'), the error will be detected and the script will terminate
• All parsing occurs before any work (mainly affects the clean step)
• If any arguments follow the final switch, the error will be detected and the script will terminate

Additionally, there was a conflict between the usage statement and logic, in that the usage statement referred to a positional argument CUSTOM_CMAKE_CXX_FLAGS=x, but the actual parser looked for an argument CUSTOM_BUILD_FLAGS=x and set a variable CUSTOM_CMAKE_CXX_FLAG. The usage statement is updated to reflect the argument.

Testing performed includes:

• All arguments and the variables they set
• Conflicting arguments result in errors
• Default values are used if no value is set for BUILD_TYPE or MAC_ARCH
• Non-switch arguments following switches result in errors
• ./build.sh noroot -DBUILD_UNIT_TESTS=NO -DBUILD_FUNC_TESTS=NO builds successfully

[ThomsonTan]

[lalitb]

@DSMitten - while looks unlikely, can you ensure that this doesn't break the existing scripts which internally invoke build.sh . As here -

cpp_client_telemetry/build-docker.sh

Line 15 in c380bc8

docker run -v `pwd`:/build -w /build $IMAGE_NAME ./build.sh $BUILD_TYPE