OpenBLAS: Fix cross-compilation detection for source build. (JuliaLang#56139)

We may be cross-compiling Linux-to-Linux, in which case `BUILD_OS` ==
`OS`, so look at `XC_HOST` to determine whether we're cross compiling.

Author: maleadt

deps/openblas.mk

@@ -43,7 +43,7 @@ OPENBLAS_FFLAGS := $(JFFLAGS) $(USE_BLAS_FFLAGS)
 OPENBLAS_CFLAGS := -O2
 
 # Decide whether to build for 32-bit or 64-bit arch
-ifneq ($(BUILD_OS),$(OS))
+ifneq ($(XC_HOST),)
 OPENBLAS_BUILD_OPTS += OSNAME=$(OS) CROSS=1 HOSTCC=$(HOSTCC) CROSS_SUFFIX=$(CROSS_COMPILE)
 endif
 ifeq ($(OS),WINNT)