bug#16930: [ac02f3a7] fails to build without HAVE_WINDOW_SYSTEM

[Ivan Shmakov <ivan@siamics.net>]

Package:  emacs

	As of ac02f3a7, Emacs fails to build with the following error.

gcc -std=gnu99 -c  -Demacs  -I. -I../../src -I../lib -I../../src/../lib        -isystem /usr/include/libxml2           -MMD -MF deps/font.d -MP -isystem /usr/include/p11-kit-1   -pthread -isystem /usr/include/glib-2.0 -isystem /usr/lib/x86_64-linux-gnu/glib-2.0/include   -W -Wabi -Waddress -Waggressive-loop-optimizations -Wall -Warray-bounds -Wattributes -Wbuiltin-macro-redefined -Wcast-align -Wchar-subscripts -Wclobbered -Wcomment -Wcomments -Wcoverage-mismatch -Wcpp -Wdeprecated -Wdeprecated-declarations -Wdisabled-optimization -Wdiv-by-zero -Wdouble-promotion -Wempty-body -Wendif-labels -Wenum-compare -Wextra -Wformat-contains-nul -Wformat-extra-args -Wformat-security -Wformat-y2k -Wformat-zero-length -Wfree-nonheap-object -Wignored-qualifiers -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Winit-self -Wint-to-pointer-cast -Winvalid-memory-model -Winvalid-pch -Wmain -Wmaybe-uninitialized -Wmissing-braces -Wmissing-declarations -Wmissing-include-dirs -Wmissing-parameter-type -Wmissing-prototypes -Wmultichar -Wnarrowing -Wnonnull -Wnormalized=nfc -Wold-style-declaration -Wold-style-definition -Woverflow -Woverride-init -Wpacked -Wpacked-bitfield-compat -Wparentheses -Wpointer-arith -Wpointer-sign -Wpointer-to-int-cast -Wpragmas -Wreturn-local-addr -Wreturn-type -Wsequence-point -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-prototypes -Wsuggest-attribute=format -Wtrampolines -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused -Wunused-but-set-parameter -Wunused-but-set-variable -Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-macros -Wunused-result -Wunused-value -Wunused-variable -Wvarargs -Wvariadic-macros -Wvector-operation-performance -Wvolatile-register-var -Wwrite-strings -fdiagnostics-show-option -funit-at-a-time -Wno-missing-field-initializers -Wno-sign-compare -Wno-type-limits -Wno-switch -Wno-unused-parameter -Wno-format-nonliteral -Wno-logical-op -Wno-unused-variable -Wno-unused-but-set-variable -Wno-unused-but-set-parameter -Werror -g3 -O2  ../../src/font.c
../../src/font.c: In function 'Fframe_font_cache':
../../src/font.c:4853:3: error: implicit declaration of function 'FRAME_DISPLAY_INFO' [-Werror=implicit-function-declaration]
   return FRAME_DISPLAY_INFO (decode_live_frame (frame))->name_list_element;
   ^
../../src/font.c:4853:56: error: invalid type argument of '->' (have 'int')
   return FRAME_DISPLAY_INFO (decode_live_frame (frame))->name_list_element;
                                                        ^
../../src/font.c:4854:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1: all warnings being treated as errors
make[1]: *** [font.o] Error 1

	As all the other invocations of FRAME_DISPLAY_INFO () seem to be
	guarded with #ifdef HAVE_WINDOW_SYSTEM, I’d rather assume that
	the whole DEFUN ("frame-font-cache", …) should also be within
	such guards, along with the reference to Sframe_font_cache
	within syms_of_font ().

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