We only need to base it on the compiler in config.h. config.h activates the code path guarded by HasNEON(). The source file that actially provides the NEON implementation will be compiled with -fpu=neon or -march=armv8-a.
Since we are providing the specialized implementation in a sequestered source file (and not a header file), we can probably avoid the defines like CRYPTOPP_ARM_NEON_AVAILABLE altogether.
Jeffrey Walton