Replace defined(COMPILER_GCC) with defined(__GNUC__).

The preprocessor expression defined(COMPILER_GCC) is used in code copied
from the Chromium project, but COMPILER_GCC is not defined by GCC itself,
but by the following expression in Chromium's build_config.h:

#if defined(__GNUC__)
#define COMPILER_GCC 1
#elif // ...

It must therefore be changed when copying the code out of the Chromium
code base.

cpp/src/phonenumbers/base/basictypes.h

@@ -205,7 +205,7 @@ struct CompileAssert {
 // is important for some other reason.)
 // Use like:
 //   int x ALLOW_UNUSED = ...;
-#if defined(COMPILER_GCC)
+#if defined(__GNUC__)
 #define ALLOW_UNUSED __attribute__((unused))
 #else
 #define ALLOW_UNUSED

tools/cpp/src/base/basictypes.h

@@ -246,7 +246,7 @@ struct CompileAssert {
 // is important for some other reason.)
 // Use like:
 //   int x ALLOW_UNUSED = ...;
-#if defined(COMPILER_GCC)
+#if defined(__GNUC__)
 #define ALLOW_UNUSED __attribute__((unused))
 #else
 #define ALLOW_UNUSED