fix build with gflags 2.1.x

[ajkr]

Builds against gflags 2.1.x built with default namespace may encounter errors like:

In file included from /home/andrewkr/local/rocksdb/tools/trace_analyzer_tool.cc:49:0:
/home/andrewkr/local/rocksdb/util/gflags_compat.h:11:26: error: ‘google’ has not been declared
 #define GFLAGS_NAMESPACE google

In gflags 2.1, the namespace changed from google to gflags. Meanwhile, it wasn't until 2.2 that the macro #define GFLAGS_NAMESPACE ... was introduced to a header file. So for 2.1 versions, this logic improperly guesses the namespace:

rocksdb/util/gflags_compat.h

Lines 8 to 12 in 57096ab

	#ifndef GFLAGS_NAMESPACE
	// in case it's not defined in old versions, that's probably because it was
	// still google by default.
	#define GFLAGS_NAMESPACE google
	#endif

.

[iFA88]

Hey @ajkr , sorry but i have still issue with this workaround -D GFLAGS_NAMESPACE=gflags :

CMake Warning:
  Manually-specified variables were not used by the project:

    GFLAGS_NAMESPACE

After build i can not use the library:
/usr/lib/x86_64-linux-gnu/librocksdb.so.6: undefined symbol: _ZN6google21ParseCommandLineFlagsEPiPPPcb

libgflags-dev has v2.1.2-4 on debian 9.

Rocksdb v6.8.1 works fine.

[ajkr]

For the workaround, -DGFLAGS_NAMESPACE should be provided as a compiler flag, not as a CMake build option.