Don't use #define CS_ENABLE_STRDUP defined().

[paulrouget]

A simple PR to suppress a warning:

warning: mjs/mjs.c:1949:5: warning: macro expansion producing 'defined' has undefined behavior [-Wexpansion-to-defined]
warning: #if CS_ENABLE_STRDUP
warning:     ^
warning: mjs/mjs.c:1192:26: note: expanded from macro 'CS_ENABLE_STRDUP'
warning: #define CS_ENABLE_STRDUP defined(__ARMCC_VERSION)
warning:                          ^
warning: mjs/mjs.c:4598:5: warning: macro expansion producing 'defined' has undefined behavior [-Wexpansion-to-defined]
warning: #if CS_ENABLE_STRDUP
warning:     ^
warning: mjs/mjs.c:1192:26: note: expanded from macro 'CS_ENABLE_STRDUP'
warning: #define CS_ENABLE_STRDUP defined(__ARMCC_VERSION)

Macro expansion producing 'defined' has undefined behavior.

As explained here:

https://stackoverflow.com/questions/42074035/how-to-deal-with-clangs-3-9-wexpansion-to-defined-warning?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa