fbgrab: fix static linking

Fixes:

  http://autobuild.buildroot.org/results/9e0/9e079632a6c0556c96ec8ed7564286aa84044bf7/
  http://autobuild.buildroot.org/results/401/401105b5e120176e78c2e5fcabe54ab0bde1bf40/
  and more

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/fbgrab/fbgrab-01-fix-static-build.patch

@@ -0,0 +1,21 @@
+Fix static build
+
+fbgrab links against libpng, which depends on libm. For shared library
+builds, there is nothing special to do about this, but for static
+library builds, it is necessary to pass -lm when linking fbgrab.
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+
+Index: b/Makefile
+===================================================================
+--- a/Makefile
++++ b/Makefile
+@@ -12,7 +12,7 @@
+ all: fbgrab fbgrab.1.gz
+ 
+ fbgrab: fbgrab.c
+-	$(CC) -g -Wall $(CFLAGS) $(LDFLAGS) $< -lpng -lz -o $@
++	$(CC) -g -Wall $(CFLAGS) $(LDFLAGS) $< -lpng -lz -lm -o $@
+ 
+ fbgrab.1.gz: fbgrab.1.man
+ 	$(GZIP) $(GZIPFLAGS) $< > $@