Correct wasm2c example Makefiles (#2426)

- Passing "-lm" into the prereq isn't the correct way add the flag. This correctly adds it to the command.

- The "rot13" example incorrectly assumed that the "rot13.h" file would be generated by the time that "main.c"
  was being compiled, however there is no rule supporting this and it would fail.
  I've also added "rot13.h" to the clean rule.

wasm2c/examples/callback/Makefile

@@ -1,12 +1,13 @@
 # Use implicit rules for compiling C files.
 CFLAGS=-I../..
+LDLIBS=-lm
 
 all: callback
 
 clean:
  rm -rf callback callback.wasm callback.c callback.h *.o
 
-callback: main.o callback.o ../../wasm-rt-impl.o ../../wasm-rt-mem-impl.o -lm
+callback: main.o callback.o ../../wasm-rt-impl.o ../../wasm-rt-mem-impl.o
 
 callback.wasm: callback.wat ../../../bin/wat2wasm
  ../../../bin/wat2wasm --debug-names $< -o $@

wasm2c/examples/fac/Makefile

@@ -1,12 +1,13 @@
 # Use implicit rules for compiling C files.
 CFLAGS=-I../..
+LDLIBS=-lm
 
 all: fac
 
 clean:
  rm -rf fac fac.wasm fac.c *.o
 
-fac: main.o fac.o ../../wasm-rt-impl.o -lm
+fac: main.o fac.o ../../wasm-rt-impl.o
 
 fac.wasm: fac.wat ../../../bin/wat2wasm
  ../../../bin/wat2wasm $< -o $@

wasm2c/examples/rot13/Makefile

@@ -1,17 +1,20 @@
 # Use implicit rules for compiling C files.
 CFLAGS=-I../..
+LDLIBS=-lm
 
 all: rot13
 
 clean:
- rm -rf rot13 rot13.wasm rot13.c *.o
+ rm -rf rot13 rot13.wasm rot13.c rot13.h *.o
 
-rot13: main.o rot13.o ../../wasm-rt-impl.o ../../wasm-rt-mem-impl.o -lm
+rot13: main.o rot13.o ../../wasm-rt-impl.o ../../wasm-rt-mem-impl.o
+
+main.o: rot13.h
 
 rot13.wasm: rot13.wat ../../../bin/wat2wasm
  ../../../bin/wat2wasm $< -o $@
 
-rot13.c: rot13.wasm ../../../bin/wasm2c
- ../../../bin/wasm2c $< -o $@ --disable-simd
+rot13.c rot13.h: rot13.wasm ../../../bin/wasm2c
+ ../../../bin/wasm2c $< -o rot13.c --disable-simd
 
 .PHONY: all clean

wasm2c/examples/threads/Makefile

@@ -1,14 +1,14 @@
 #SANITIZERS=-fsanitize=address -fsanitize=undefined -fno-sanitize-recover=all
 CFLAGS=-I../.. -g -O2 -Wall -Wextra -Wno-unused -Wno-unused-parameter -Wno-array-bounds -Wno-ignored-optimization-argument -Wno-tautological-constant-out-of-range-compare -Wno-infinite-recursion -fno-optimize-sibling-calls -frounding-math -fsignaling-nans ${SANITIZERS} -pthread
-LDFLAGS=${SANITIZERS} -pthread
+LDLIBS=${SANITIZERS} -pthread -lm
 
 all: threads
 
-threads: threads.o sample.o ../../wasm-rt-impl.o ../../wasm-rt-exceptions-impl.o -lm
-
 clean:
  rm -rf threads sample.wasm sample.c sample.h *.o ../../*.o
 
+threads: threads.o sample.o ../../wasm-rt-impl.o ../../wasm-rt-exceptions-impl.o
+
 sample.wasm: sample.wat ../../../bin/wat2wasm
  ../../../bin/wat2wasm --debug-names $< -o $@