Makefile: allow injecting prebuilt thrift compiler

This is required for Nix builds.

I also switched pwd to be resolved from PATH because /bin/pwd is not
a thing on NixOS.

Author: lf-

Makefile

@@ -63,7 +63,7 @@
 #
 
 CABAL_BIN=cabal
-PWD := $(shell /bin/pwd)
+PWD := $(shell pwd)
 
 # There's a lot of parallelism in the schema-generated code
 # If you have >=16G and >=4 cores, trying passing these:
@@ -243,13 +243,25 @@ thrift:: thrift-compiler thrift-hs
 thrift-hs:: thrift-hsthrift-hs thrift-glean-hs
 
 .PHONY: thrift-compiler
+# Allow injecting a prebuilt thrift compiler by setting THRIFT_COMPILE in
+# environment (e.g. with Nix).
+ifndef THRIFT_COMPILE
 thrift-compiler::
 	(cd hsthrift && make CABAL="$(CABAL)" compiler)
 	$(eval THRIFT_COMPILE := $$(shell $$(CABAL) -v0 list-bin exe:thrift-compiler))
+else
+thrift-compiler::
+	# no-op
+endif
 
 .PHONY: thrift-hsthrift-hs
+ifndef THRIFT_COMPILE
 thrift-hsthrift-hs::
 	(cd hsthrift && make CABAL="$(CABAL)" thrift-hs)
+else
+thrift-hsthrift-hs::
+	# no-op
+endif
 
 .PHONY: gen-schema
 gen-schema :: glean.cabal cxx-libraries