Ensure that the compilation of extensions get the same CFLAGS as the core lib. Move...

Ensure that the compilation of extensions get the same CFLAGS as the core lib. Move the feature flags enabled by --dev out of TARGET_DEBUG and into OPT_FEATURE_FLAGS (for consistency). Rename the make-internal gcov-related flags to be more descriptive. At the end of the configure script, if SQLITE_DEBUG is active then emit a note reminding the builder that performance will suffer.

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