fix C compiler warnings

[vszakats]

Fix C compiler warnings:

• -Wcast-qual
• -Wdeclaration-after-statement
• -Wmissing-noreturn
• -Wmissing-variable-declarations
• -Wsign-conversion (some)
• -Wunused-macros (most)

Also:

• cmake: enable warnings fixed in full.
• cmake: prefer -Wextra in clang. (sync with curl.)
• Makefile: also enable warnings fixed in full.
  (only those shared between clang/gcc)