CTS: Fix compile warnings.

[lakshmih]

For 32 and 64-bit Visual Studio and the Android Q NDK.

General:

• Improve variable types.
• Make casts explicit to improve readability.
• Improve printf format specifiers, favoe hex values.
• Don't warn about using the portable "strdup" on Visual Studio.
• Limit the mark pragma to Apple environments since it's only useful in XCode and causes a warning otherwise.
• Move GLES3 macro from CMakeLists.txt to test_conformance/gles/CMakeLists.txt to be more specific.
• Remove unused variables.
• Fix type narrowing caused by assigning size_ts to ints. This also better matches the types of clEnqueueNDRangeKernel's local size, global size, and global offset.

test_common/gles/helpers.cpp:

• Remove needless realWidth and realHeight variables.

test_common/harness/compat.h:

• Don't redefine EXTERN_C on Windows where it's already in the OS headers.

test_common/harness/conversions.cpp:

• Fix warning about storing -1 into unsigned long upper limits.

test_common/harness/errorHelpers.h:

• Remove the custom compile time assert in favor of the standard "static_assert".

test_common/harness/imageHelpers.cpp:

• Use the standard "static_assert" as supported by Visual Studio 2015 instead of the custom "ct_assert".

test_conformance/subgroups: Fix most compile warnings.

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