Use 'constexpr' when including pio header files from a C++ compiler

In particular this allows compile-time checks to be performed on the
contents of pio_program structs via 'static_assert'.

This could be used (for example) to confirm that multiple PIO programs
fit in a single PIO module and abort compilation if not.

Fixes #2650

Author: rhulme

tools/pioasm/c_sdk_output.cpp

@@ -79,6 +79,13 @@ struct c_sdk_output : public output_format {
         fprintf(out, "#endif\n");
         fprintf(out, "\n");
 
+        fprintf(out, "#ifdef __cplusplus\n");
+        fprintf(out, "#define PIO_CONST constexpr\n");
+        fprintf(out, "#else\n");
+        fprintf(out, "#define PIO_CONST const\n");
+        fprintf(out, "#endif\n");
+        fprintf(out, "\n");
+
         output_symbols(out, "", source.global_symbols);
 
         for (const auto &program : source.programs) {
@@ -93,7 +100,7 @@ struct c_sdk_output : public output_format {
 
             output_symbols(out, prefix, program.symbols);
 
-            fprintf(out, "static const uint16_t %sprogram_instructions[] = {\n", prefix.c_str());
+            fprintf(out, "static PIO_CONST uint16_t %sprogram_instructions[] = {\n", prefix.c_str());
             for (int i = 0; i < (int)program.instructions.size(); i++) {
                 const auto &inst = program.instructions[i];
                 if (i == program.wrap_target) {
@@ -109,7 +116,7 @@ struct c_sdk_output : public output_format {
             fprintf(out, "\n");
 
             fprintf(out, "#if !PICO_NO_HARDWARE\n");
-            fprintf(out, "static const struct pio_program %sprogram = {\n", prefix.c_str());
+            fprintf(out, "static PIO_CONST struct pio_program %sprogram = {\n", prefix.c_str());
             fprintf(out, "    .instructions = %sprogram_instructions,\n", prefix.c_str());
             fprintf(out, "    .length = %d,\n", (int) program.instructions.size());
             fprintf(out, "    .origin = %d,\n", program.origin.get());