meson: rename cpp_cmd to cxx_cmd

cpp is a really bad alias for c++ because it's also the name of the
preprocessor. Let's rename the variable.

meson.build

@@ -278,9 +278,9 @@ cxx = find_program('c++', required : false)
 if cxx.found()
         #  Used only for tests
         add_languages('cpp')
-        cpp_cmd = ' '.join(meson.get_compiler('cpp').cmd_array())
+        cxx_cmd = ' '.join(meson.get_compiler('cpp').cmd_array())
 else
-        cpp_cmd = ''
+        cxx_cmd = ''
 endif
 
 want_ossfuzz = get_option('oss-fuzz')

test/fuzz/meson.build

@@ -9,7 +9,7 @@ sanitize_address = custom_target(
                    'fuzzers',
                    '-Db_lundef=false -Db_sanitize=address',
                    ' '.join(cc.cmd_array()),
-                   cpp_cmd])
+                   cxx_cmd])
 
 sanitizers = [['address', sanitize_address]]