MB-7810: Add -Wunused-variable to compiler warnings list

Change-Id: I3352562a066581dd336f20e3a5b49f847eea60d4
Reviewed-on: http://review.couchbase.org/24797
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>

m4/couchbase.m4

@@ -63,7 +63,7 @@ AC_DEFUN([COUCHBASE_GENERIC_COMPILER], [
   GCC_C89=-std=c89
   GCC_C99=-std=gnu99
   GCC_LDFLAGS=""
-  GCC_CPP_WARNINGS="-Wall -pedantic -Wshadow -fdiagnostics-show-option -Wformat -fno-strict-aliasing -Wno-strict-aliasing -Wextra"
+  GCC_CPP_WARNINGS="-Wall -pedantic -Wshadow -fdiagnostics-show-option -Wformat -fno-strict-aliasing -Wno-strict-aliasing -Wextra -Wunused-variable"
   GCC_C_COMPILER_WARNINGS="-Wundef -Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls -Wmissing-declarations -Wcast-align"
   GCC_CXX_COMPILER_WARNINGS="-std=gnu++98 -Woverloaded-virtual -Wnon-virtual-dtor -Wctor-dtor-privacy -Wno-long-long -Wno-redundant-decls"