build: added default linker flag in case pkg-config doesn't have z3 i…

…n its database, such as in NIX; fixes opp_env issue #24 (omnetpp/opp_env#24)

src/makefrag

@@ -80,11 +80,10 @@ endif
 WITH_Z3 := $(shell (cd .. && $(FEATURETOOL) -q isenabled Z3GateSchedulingConfigurator && echo enabled) )
 ifeq ($(WITH_Z3), enabled)
   ifeq ($(HAVE_PKGCFG), yes)
-    HAVE_Z3 := $(shell $(PKGCFG) --exists z3 && echo yes || echo no)
-    ifeq ($(HAVE_Z3), yes)
-      LIBS += $(shell $(PKGCFG) --libs z3)
-      CFLAGS += $(shell $(PKGCFG) --cflags z3) -DHAVE_Z3
-    endif
+    Z3_LIBS := $(shell $(PKGCFG) --libs z3)
+    Z3_LIBS ?= -lz3     # default in case pkg-config doesn't have z3 in its database (such as in NIX)
+    LIBS += $(Z3_LIBS)
+    CFLAGS += $(shell $(PKGCFG) --cflags z3) -DHAVE_Z3
   endif
 endif