Quiet warnings for stricter compilers configurations (#84)

Fix warnings from -Wcast-qual and -Wstrict-prototypes.

A few other warning fixes and documentation improvements are added.

Also fixes Makefile so the before-release build script can invoke both gcc and clang. That script is here: laurencelundblade/qdv@536f083

This is an alternative to #79. It does not fix warnings for -Wbad-function-cast
because those fixes cause warnings with other compilers (MacOS clang). This doesn't enable GitHub automated build checks. The plan is to do that separately.
diff --git a/test/qcbor_encode_tests.c b/test/qcbor_encode_tests.c
index f9d8507..c3e7315 100644
--- a/test/qcbor_encode_tests.c
+++ b/test/qcbor_encode_tests.c
@@ -116,10 +116,10 @@
                                 struct UBCompareDiagnostic *pDiag) {
    size_t i;
    for(i = 0; i < Actual.len; i++) {
-      if(((uint8_t *)Actual.ptr)[i] != ((uint8_t *)Expected.ptr)[i]) {
+      if(((const uint8_t *)Actual.ptr)[i] != ((const uint8_t *)Expected.ptr)[i]) {
          if(pDiag) {
-            pDiag->uActual   = ((uint8_t *)Actual.ptr)[i];
-            pDiag->uExpected = ((uint8_t *)Expected.ptr)[i];
+            pDiag->uActual   = ((const uint8_t *)Actual.ptr)[i];
+            pDiag->uExpected = ((const uint8_t *)Expected.ptr)[i];
             pDiag->uOffset   = i;
          }
          // Cast to int is OK as this is only a diagnostic and the sizes