Makefile: fix $PWD expansion

Makefile

@@ -29,7 +29,7 @@ SOURCES := $(shell find src -name '*.cr')
 SPEC_SOURCES := $(shell find spec -name '*.cr')
 FLAGS := $(if $(release),--release )$(if $(stats),--stats )$(if $(progress),--progress )$(if $(threads),--threads $(threads) )$(if $(debug),-d )$(if $(static),--static )$(if $(LDFLAGS),--link-flags="$(LDFLAGS)" )
 SPEC_FLAGS := $(if $(verbose),-v )$(if $(junit_output),--junit_output $(junit_output) )
-EXPORTS := $(if $(release),,CRYSTAL_CONFIG_PATH=$PWD/src)
+EXPORTS := $(if $(release),,CRYSTAL_CONFIG_PATH="$(PWD)/src")
 SHELL = sh
 LLVM_CONFIG_FINDER := \
   [ -n "$(LLVM_CONFIG)" ] && command -v "$(LLVM_CONFIG)" || \