Force all OpenLibm fenv-related functions to be globally exported

We do this in the same manner that OpenLibm itself does it; by defining
the `__fenv_static` identifier to be `OLM_DLLEXPORT`.  `static` ==
`DLLEXPORT`.  Not confusing at all, right?  ;)

We do it in this manner to avoid needing to change the default behavior
for non-Julia users of OpenLibm, which do exist.  This way, a default
build of OpenLibm has the same behavior as it always has.

deps/openlibm.mk

@@ -7,9 +7,13 @@ $(eval $(call git-external,openlibm,OPENLIBM,,,$(BUILDDIR)))
 
 OPENLIBM_FLAGS := ARCH="$(ARCH)" REAL_ARCH="$(MARCH)" CC="$(CC)" FC="$(FC)" AR="$(AR)" OS="$(OS)" USECLANG=$(USECLANG) USEGCC=$(USEGCC)
 
+# We do this awful awful thing where we override `__fenv_static` to be `OLM_DLLEXPORT` so that the fenv methods
+# like `fesetround()` are exported on all platforms.
+OPENLIBM_CFLAGS := "-D__fenv_static=OLM_DLLEXPORT"
+
 ifneq ($(USE_BINARYBUILDER_OPENLIBM), 1)
 $(BUILDDIR)/$(OPENLIBM_SRC_DIR)/build-compiled: $(BUILDDIR)/$(OPENLIBM_SRC_DIR)/source-extracted
-	$(MAKE) -C $(dir $<) $(OPENLIBM_FLAGS) $(MAKE_COMMON)
+	CFLAGS_add="$(OPENLIBM_CFLAGS)" $(MAKE) -C $(dir $<) $(OPENLIBM_FLAGS) $(MAKE_COMMON)
 	echo 1 > $@
 
 $(eval $(call staged-install, \