tools/: Link curve_keygen with a c++ linker

When trying to linking curve_keygen as if it was a C program, linking
fails (at least with gcc 4.8 on Debian testing) because it can't find
C++ symbols. Linking with g++ instead fixes the issue.

To achieve this, the source is renamed to curve_keygen.cpp, and
tools/Makefile.am is updated accordingly.
Signed-off-by: Gergely Nagy <algernon@balabit.hu>

tools/Makefile.am

-EXTRA_DIST = curve_keygen.c 
+EXTRA_DIST = curve_keygen.cpp
 
 INCLUDES = -I$(top_srcdir)/include
 
 bin_PROGRAMS = curve_keygen
 
 curve_keygen_LDADD = $(top_builddir)/src/libzmq.la
-curve_keygen_SOURCES = curve_keygen.c
+curve_keygen_SOURCES = curve_keygen.cpp