Bug 783482 - Fail to link invert-svg in jhbuild prefix on FreeBSD...

... because LDFLAGS is ignored.
Firstly let's make sure that invert-svg build uses LDFLAGS_FOR_BUILD and
CFLAGS_FOR_BUILD; secondly, default these to LDFLAGS and CFLAGS
respectively when not-cross-compiling, unless values are explicitly set.