gdb: enable -Wmissing-prototypes warning

While compiling with clang, I noticed it didn't catch cases where my
function declaration didn't match my function definition.  This is
normally caught by gcc with -Wmissing-declarations.

On clang, this is caught by -Wmissing-prototypes instead.

Note that on gcc, -Wmissing-prototypes also exists, but is only valid
for C and Objective-C.  It gets correctly rejected by the configure
script since gcc rejects it with:

    cc1plus: error: command line option '-Wmissing-prototypes' is valid for C/ObjC but not for C++ -Werror

So this warning flag ends up not used for gcc (which is what we want).

gdb/ChangeLog:

	* configure: Re-generate.

gdbserver/ChangeLog:

	* configure: Re-generate.

gdbsupport/ChangeLog:

	* configure: Re-generate.
	* warning.m4: Enable -Wmissing-prototypes.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-11  Simon Marchi  <simon.marchi@efficios.com>
+
+	* configure: Re-generate.
+
 2020-03-11  Tom Tromey  <tromey@adacore.com>
 
 	* ada-typeprint.c (print_choices): Fix comment.

gdb/configure

@@ -16323,6 +16323,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
+-Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 

gdbserver/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-11  Simon Marchi  <simon.marchi@efficios.com>
+
+	* configure: Re-generate.
+
 2020-03-06  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* .dir-locals.el: New file.

gdbserver/configure

@@ -9616,6 +9616,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
+-Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 

gdbsupport/ChangeLog

@@ -1,3 +1,8 @@
+2020-03-11  Simon Marchi  <simon.marchi@efficios.com>
+
+	* configure: Re-generate.
+	* warning.m4: Enable -Wmissing-prototypes.
+
 2020-03-08  Tom Tromey  <tom@tromey.com>
 
 	* gdb_binary_search.h: Fix two typos.

gdbsupport/configure

@@ -10874,6 +10874,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
+-Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "
 

gdbsupport/warning.m4

@@ -51,6 +51,7 @@ build_warnings="-Wall -Wpointer-arith \
 -Wdeprecated-copy-dtor \
 -Wredundant-move \
 -Wmissing-declarations \
+-Wmissing-prototypes \
 -Wstrict-null-sentinel \
 "