Parametrize WASM compilation process (#87)

* Makefile: extract compilation flags to variables

Previously, the WASM compilation step was hard to adjust for different
systems, as it required re-writing the actual rule.

This change extracts all the flags into make variables,
and separates them into system- and project-specific ones.

The system-specific ones are expected to change from machine to machine,
and now can be easily overridden:

    $ make WASM_CC=clang WASM_CFLAGS=--target=wasm32-wasi …

The project-specific flags are also extracted to variables,
although at this point this is mainly for consistency sake.

* Makefile: parametrize all binaries

Similarly to previous change,
this allows for all binary paths to be easily adjusted.

* Makefile: polish

Tiny leftover nitpicks:

- Mark `optimize` and `clean` targets as phony
- Use separate target for `lib/` directory creation
- Use make's built-in `$(RM)` command

Author: khardix

Makefile

@@ -1,16 +1,33 @@
-lslib/lexer.wat: lib/lexer.wasm
-	../wabt/bin/wasm2wat lib/lexer.wasm -o lib/lexer.wat
+# These flags depend on the system and may be overridden
+WASM_CC := ../wasi-sdk-12.0/bin/clang
+WASM_CFLAGS := --sysroot=../wasi-sdk-12.0/share/wasi-sysroot
+WASM_LDFLAGS := -nostartfiles
 
-lib/lexer.wasm: include-wasm/cjs-module-lexer.h src/lexer.c
-	@mkdir -p lib
-	../wasi-sdk-12.0/bin/clang src/lexer.c -I include-wasm --sysroot=../wasi-sdk-12.0/share/wasi-sysroot -o lib/lexer.wasm -nostartfiles \
-	-Wl,-z,stack-size=13312,--no-entry,--compress-relocations,--strip-all,--export=__heap_base,\
-	--export=parseCJS,--export=sa,--export=e,--export=re,--export=es,--export=ee,--export=rre,--export=ree,--export=res,--export=ru,--export=us,--export=ue \
-	-Wno-logical-op-parentheses -Wno-parentheses \
-	-Oz
+WASM2WAT := ../wabt/bin/wasm2wat
+WASM_OPT := ../binaryen/bin/wasm-opt
+
+# These are project-specific and are expected to be kept intact
+WASM_EXTRA_CFLAGS := -I include-wasm/ -Wno-logical-op-parentheses -Wno-parentheses -Oz
+WASM_EXTRA_LDFLAGS := -Wl,-z,stack-size=13312,--no-entry,--compress-relocations,--strip-all
+WASM_EXTRA_LDFLAGS += -Wl,--export=__heap_base,--export=parseCJS,--export=sa
+WASM_EXTRA_LDFLAGS += -Wl,--export=e,--export=re,--export=es,--export=ee
+WASM_EXTRA_LDFLAGS += -Wl,--export=rre,--export=ree,--export=res,--export=ru,--export=us,--export=ue
+
+.PHONY: optimize clean
+
+lib/lexer.wat: lib/lexer.wasm
+	$(WASM2WAT) lib/lexer.wasm -o lib/lexer.wat
+
+lib/lexer.wasm: include-wasm/cjs-module-lexer.h src/lexer.c | lib/
+	$(WASM_CC) $(WASM_CFLAGS) $(WASM_EXTRA_CFLAGS) \
+		src/lexer.c -o lib/lexer.wasm \
+		$(WASM_LDFLAGS) $(WASM_EXTRA_LDFLAGS)
+
+lib/:
+	@mkdir -p $@
 
 optimize: lib/lexer.wasm
-	../binaryen/bin/wasm-opt -Oz lib/lexer.wasm -o lib/lexer.wasm
+	$(WASM_OPT) -Oz lib/lexer.wasm -o lib/lexer.wasm
 
 clean:
-	rm lib/*
+	$(RM) lib/*