Build: Add SerenityOS to list of compatible systems

This is required by the build system to spit out a library with
the correct name/platform.

engine/common/build.c

@@ -95,6 +95,8 @@ const char *Q_buildos( void )
 	osname = "DOS4GW";
 #elif XASH_HAIKU
 	osname = "haiku";
+#elif XASH_SERENITY
+	osname = "serenityos";
 #else
 #error "Place your operating system name here! If this is a mistake, try to fix conditions above and report a bug"
 #endif

public/build.h

@@ -74,6 +74,7 @@ For more information, please refer to <http://unlicense.org/>
 #undef XASH_RISCV_DOUBLEFP
 #undef XASH_RISCV_SINGLEFP
 #undef XASH_RISCV_SOFTFP
+#undef XASH_SERENITY
 #undef XASH_WIN32
 #undef XASH_WIN64
 #undef XASH_X86
@@ -125,6 +126,9 @@ For more information, please refer to <http://unlicense.org/>
 #elif defined __HAIKU__
 	#define XASH_HAIKU 1
 	#define XASH_POSIX 1
+#elif defined __serenity__
+	#define XASH_SERENITY 1
+	#define XASH_POSIX 1
 #else
 #error "Place your operating system name here! If this is a mistake, try to fix conditions above and report a bug"
 #endif