build: Treat implicit function declarations in C as errors

Building with CFLAGS="-std=c11" using GCC exposed that certain
function extensions to the C standard could have their pointer return
values truncated.

E.g. the POSIX strdup extension was unknown to gcc when using "-std=c11".
GCC assumed it would be a function that returned an "int". That would
then fail if the string pointer required > 32 bits.

C99 removed support for implicit functions declarations. Some compilers,
e.g. like gcc, will only warn about it. Clang has changed it to an
error in version 16.

strdup for example has been added to C23.

Workaround for #77

CMakeLists.txt

@@ -73,7 +73,7 @@ if(MSVC)
         endforeach()
     endif()
 else()
-    add_compile_options(-Wextra -Wall)
+    add_compile_options(-Wextra -Wall $<$<COMPILE_LANGUAGE:C>:-Werror=implicit-function-declaration>)
 
     # Enable large file support on 32-bit systems.
     add_definitions(

deps/libMXF/CMakeLists.txt

@@ -74,7 +74,7 @@ if(MSVC)
         endforeach()
     endif()
 else()
-    add_compile_options(-Wextra -Wall)
+    add_compile_options(-Wextra -Wall $<$<COMPILE_LANGUAGE:C>:-Werror=implicit-function-declaration>)
 
     # Enable large file support on 32-bit systems.
     add_definitions(

deps/libMXFpp/CMakeLists.txt

@@ -59,7 +59,7 @@ if(MSVC)
         endforeach()
     endif()
 else()
-    add_compile_options(-Wextra -Wall)
+    add_compile_options(-Wextra -Wall $<$<COMPILE_LANGUAGE:C>:-Werror=implicit-function-declaration>)
 
     # Enable large file support on 32-bit systems.
     add_definitions(