The Modelica/Resources/BuildProjects/gcc/Makefile does not handle missing installation directory, which was the case when I downloaded the latest ModelicaStandardLibrary-master.
One possible remedy is to complete the all target and add a new target for the directory:

all: clean ../../Library/$(TARGETDIR)
	$(MAKE) libModelicaExternalC.a libModelicaStandardTables.a libModelicaIO.a libModelicaMatIO.a libzlib.a

../../Library/$(TARGETDIR):
	mkdir -p $@