Compiler.h: add macro GCC_MAKE_VERSION()
util/Manual: reimplement GCC_CHECK_VERSION() using GCC_MAKE_VERSION()
Compiler.h: add macro GCC_OLDER_THAN()
Compiler.h: add macro CLANG_CHECK_VERSION()
util/ASCII: fix indent
DatabaseGlue: convert nullptr check to assertion
util/{ASCII,UriUtil}, ...: work around -Wtautological-pointer-compare

New in clang 3.6.
db/{simple,proxy}, ...: add "override" keywords

Fixes -Winconsistent-missing-override (clang 3.6).
db/proxy, output/shout: fix implicit nullptr/bool conversion

Return false on error, not nullptr.