tools: invert-svg is to be built natively for the build OS.

Partially reverts commit 84439a8748.
The fact that tools/invert-svg was kind of manually compiled was on
purpose. Using the automake syntax with any of the *_PROGRAMS compiles
for the host OS, but this tool is only for compile-time.
This fixes cross-compilation.

As for compute_svg_viewbox, let's not build it for now because its
purpose was to help proper extraction of SVG icons from a single SVG
file. Unfortunately librsvg was not providing acceptable results anyway
so right now all SVG icons are manually exported. We keep the code for
future use when the library will improve.

tools/Makefile.am

@@ -22,10 +22,7 @@ bin_PROGRAMS = gimptool-2.0
 
 endif
 
-noinst_PROGRAMS = \
-	test-clipboard	\
-	invert-svg	\
-	compute-svg-viewbox
+noinst_PROGRAMS = test-clipboard
 
 EXTRA_PROGRAMS = \
 	kernelgen
@@ -46,18 +43,19 @@ test_clipboard_SOURCES = test-clipboard.c
 test_clipboard_LDADD = $(GTK_LIBS)
 
 
-invert_svg_SOURCES = invert-svg.c
-
-invert_svg_CFLAGS = $(SVG_CFLAGS) $(GLIB_CFLAGS) $(GIO_CFLAGS)
-
-invert_svg_LDADD = $(SVG_LIBS) $(GLIB_LIBS) $(GIO_LIBS)
+invert-svg$(BUILD_EXEEXT): invert-svg.c
+	$(CC_FOR_BUILD) -o $@ $< $(NATIVE_GLIB_LIBS) $(NATIVE_GLIB_CFLAGS)
 
 
-compute_svg_viewbox_SOURCES = compute-svg-viewbox.c
+# compute_svg_viewbox is not built or used because librsvg is just too buggy
+# right now. But we keep the code around. The goal of this build tool will be
+# to be able to extract SVG icons from a single SVG file at build time, rather
+# than having to export and commit them manually.
+#compute_svg_viewbox_SOURCES = compute-svg-viewbox.c
 
-compute_svg_viewbox_CFLAGS = $(SVG_CFLAGS)
+#compute_svg_viewbox_CFLAGS = $(SVG_CFLAGS)
 
-compute_svg_viewbox_LDADD = $(SVG_LIBS)
+#compute_svg_viewbox_LDADD = $(SVG_LIBS)
 
 
 AM_CPPFLAGS = \
@@ -89,3 +87,6 @@ EXTRA_DIST = \
 	defcheck.py	\
 	gimp-mkenums	\
 	gimppath2svg.py
+
+# Build tools which must be built for the host platform.
+all: invert-svg$(BUILD_EXEEXT)