Bug#57995: Compiler flag change build error on OSX 10.4: my_getncpus.c
Bug#57996: Compiler flag change build error on OSX 10.5 : bind.c
Bug#57994: Compiler flag change build error : my_redel.c
Bug#57993: Compiler flag change build error on FreeBsd 7.0 : regexec.c
Bug#57992: Compiler flag change build error on FreeBsd : mf_keycache.c
Bug#57997: Compiler flag change build error on OSX 10.6: debug_sync.cc

Fix assorted compiler generated warnings.

cmd-line-utils/readline/bind.c:
  Bug#57996: Compiler flag change build error on OSX 10.5 : bind.c
  
  Initialize variable to work around a false positive warning.
include/m_string.h:
  Bug#57994: Compiler flag change build error : my_redel.c
  
  The expansion of stpcpy (in glibc) causes warnings if the
  return value of strmov is not being used. Since stpcpy is
  a GNU extension and the expansion ends up using a built-in
  provided by GCC, use the compiler provided built-in directly
  when possible.
include/my_compiler.h:
  Define a dummy MY_GNUC_PREREQ when not compiling with GCC.
libmysql/libmysql.c:
  Bug#58057: 5.1 libmysql/libmysql.c unused variable/compile failure
  
  Variable might not be used in some cases. So, tag it as unused.
mysys/mf_keycache.c:
  Bug#57992: Compiler flag change build error on FreeBsd : mf_keycache.c
  
  Use UNINIT_VAR to work around a false positive warning.
mysys/my_getncpus.c:
  Bug#57995: Compiler flag change build error on OSX 10.4: my_getncpus.c
  
  Declare variable in the same block where it is used.
regex/regexec.c:
  Bug#57993: Compiler flag change build error on FreeBsd 7.0 : regexec.c
  
  Work around a compiler bug which causes the cast to not be enforced.
sql/debug_sync.cc:
  Bug#57997: Compiler flag change build error on OSX 10.6: debug_sync.cc
  
  Use UNINIT_VAR to work around a false positive warning.
sql/handler.cc:
  Use UNINIT_VAR to work around a false positive warning.
sql/slave.cc:
  Use UNINIT_VAR to work around a false positive warning.
sql/sql_partition.cc:
  Use UNINIT_VAR to work around a false positive warning.
storage/myisam/ft_nlq_search.c:
  Use UNINIT_VAR to work around a false positive warning.
storage/myisam/mi_create.c:
  Use UNINIT_VAR to work around a false positive warning.
storage/myisammrg/myrg_open.c:
  Use UNINIT_VAR to work around a false positive warning.
tests/mysql_client_test.c:
  Change function to take a pointer to const, no need for a cast.