Remove C-specific warnings from common warning set

Remove C-specific warnings from common warning set

Whoops, these are C specific, but I somehow missed the warnings before:

  cc1plus: warning: command line option ‘-Wmissing-prototypes’ is valid for C/ObjC but not for C++ [enabled by default]
  cc1plus: warning: command line option ‘-Wdeclaration-after-statement’ is valid for C/ObjC but not for C++ [enabled by default]
  cc1plus: warning: command line option ‘-Wmissing-parameter-type’ is valid for C/ObjC but not for C++ [enabled by default]
  cc1plus: warning: command line option ‘-Wold-style-declaration’ is valid for C/ObjC but not for C++ [enabled by default]
  cc1plus: warning: command line option ‘-Wold-style-definition’ is valid for C/ObjC but not for C++ [enabled by default]

gdb/ChangeLog:
2015-03-07  Pedro Alves  <palves@redhat.com>

	* configure.ac (build_warnings): Move -Wmissing-prototypes
	-Wdeclaration-after-statement -Wmissing-parameter-type
	-Wold-style-declaration -Wold-style-definition to the C-specific
	set.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2015-03-07  Pedro Alves  <palves@redhat.com>

	* configure.ac (build_warnings): Move
	-Wdeclaration-after-statement to the C-specific set.
	* configure: Regenerate.