ci: Force unsigned char usage for the 32bit build

Since C leaves whether char is signed or unsigned up to the implementer,
there are different defaults on different architectures.

Forcing unsigned char for one of our CI builds should help catch these
issues moving forward.

CMakeLists.txt

@@ -253,6 +253,10 @@ option(
 if(TRAVIS_CI_BUILD)
   message(STATUS "Travis CI build enabled.")
   add_definitions(-Werror)
+  if(DEFINED ENV{BUILD_32BIT})
+    # Get some test coverage for unsigned char
+    add_definitions(-funsigned-char)
+  endif()
 endif()
 
 if(CMAKE_BUILD_TYPE MATCHES Debug)