Added stricter error checks by the compiler, and adapted all impacted code.

Primarily -
* Added "static" to static functions.
* Added proper signature with "void" to functions without arguments.
* Marked unused arguments with "(void)".
* Removed entirely unused static functions.
* Added "const" to preserve const-correctness.
* Added function prototypes for external functions.

test/targets/gcc_auto_stdint.yml

@@ -10,6 +10,22 @@ compiler:
     - '-Wno-address'
     - '-std=c99'
     - '-pedantic'
+    - '-Wextra'
+    - '-Werror'
+    - '-Wpointer-arith'
+    - '-Wcast-align'
+    - '-Wwrite-strings'
+    - '-Wswitch-default'
+    - '-Wunreachable-code'
+    - '-Winit-self'
+    - '-Wlogical-op'
+    - '-Wmissing-field-initializers'
+    - '-Wno-unknown-pragmas'
+    - '-Wjump-misses-init'
+    - '-Wstrict-prototypes'
+    - '-Wundef'
+    - '-Wunsafe-loop-optimizations'
+    - '-Wold-style-definition'
   includes:
     prefix: '-I'
     items: