cmake: add various warning flags like autotools has

cmake: add various warning flags like autotools has

This is mostly to match autotools and reduce amount mixiing declarations
and code.

Added:
- -Wextra (the same as -W), -Wno-unused-parameter -Wstrict-aliasing
- -fno-strict-aliasing (gcc 2.9.5+)
- -Winit-self -Wmissing-field-initializers -Wdeclaration-after-statement (4.0+)
- -Waddress -Wno-unused-function -Wnormalized=id -Woverride-init (4.2+)
- -Wlogical-op (4.5+)

Removed:
- -Wformat (include in -Wall)

Plus use CMAKE_C_COMPILER_ID over CMAKE_COMPILER_IS_GNUCC, as
cmake-variables(7) suggesting, and add common GNUC/CLANG variables.

v2: drop checks for flags, since add_compiler_flags() will check if such
flags exists anyway (but just to note, gcc ignores non existing warning
flags by default).