Use defined(...) when testing for defined-ness of macros

Visual Studio rightfully complains that these macros, the value of which was
being used, are not defined. We don't actually care about the value, just
definedness matters.

Author: tautschnig

src/util/config.cpp

@@ -1217,33 +1217,33 @@ irep_idt configt::this_architecture()
 
   #ifdef __alpha__
   this_arch="alpha";
-  #elif __armel__
+  #elif defined(__armel__)
   this_arch="armel";
-  #elif __aarch64__
+  #elif defined(__aarch64__)
   this_arch="arm64";
-  #elif __arm__
+  #elif defined(__arm__)
     #ifdef __ARM_PCS_VFP
     this_arch="armhf"; // variant of arm with hard float
     #else
     this_arch="arm";
     #endif
-  #elif __mipsel__
+  #elif defined(__mipsel__)
     #if _MIPS_SIM==_ABIO32
     this_arch="mipsel";
     #elif _MIPS_SIM==_ABIN32
     this_arch="mipsn32el";
     #else
     this_arch="mips64el";
     #endif
-  #elif __mips__
+  #elif defined(__mips__)
     #if _MIPS_SIM==_ABIO32
     this_arch="mips";
     #elif _MIPS_SIM==_ABIN32
     this_arch="mipsn32";
     #else
     this_arch="mips64";
     #endif
-  #elif __powerpc__
+  #elif defined(__powerpc__)
     #if defined(__ppc64__) || defined(__PPC64__) || \
         defined(__powerpc64__) || defined(__POWERPC64__)
       #ifdef __LITTLE_ENDIAN__
@@ -1254,33 +1254,33 @@ irep_idt configt::this_architecture()
     #else
     this_arch="powerpc";
     #endif
-  #elif __sparc__
+  #elif defined(__sparc__)
     #ifdef __arch64__
     this_arch="sparc64";
     #else
     this_arch="sparc";
     #endif
-  #elif __ia64__
+  #elif defined(__ia64__)
   this_arch="ia64";
-  #elif __s390x__
+  #elif defined(__s390x__)
   this_arch="s390x";
-  #elif __s390__
+  #elif defined(__s390__)
   this_arch="s390";
-  #elif __x86_64__
+  #elif defined(__x86_64__)
     #ifdef __ILP32__
     this_arch="x32"; // variant of x86_64 with 32-bit pointers
     #else
     this_arch="x86_64";
     #endif
-  #elif __i386__
+  #elif defined(__i386__)
   this_arch="i386";
-  #elif _WIN64
+  #elif defined(_WIN64)
   this_arch="x86_64";
-  #elif _WIN32
+  #elif defined(_WIN32)
   this_arch="i386";
-  #elif __hppa__
+  #elif defined(__hppa__)
   this_arch="hppa";
-  #elif __sh__
+  #elif defined(__sh__)
   this_arch="sh4";
   #else
   // something new and unknown!