Create `configure` script to generate a cross-platform Makefile

The previous Makefile only worked on OS X. The dynamically generated
Makefile (from `Makefile.in`) should now work on multiple platforms
(tested on OS X and Ubuntu).

.gitignore

@@ -5,4 +5,5 @@
 /sources
 /cache
 /libexec/*.dylib
+/src/Makefile
 /src/*.o

src/Makefile

@@ -1,10 +0,0 @@
-SHOBJ_LDFLAGS = -dynamiclib
-
-.c.o:
-	$(CC) $(CFLAGS) -c -o $@ $<
-
-../libexec/rbenv-realpath.dylib: realpath.o
-	$(CC) $(CFLAGS) $(SHOBJ_LDFLAGS) -o $@ realpath.o
-
-clean:
-	rm -f *.o ../libexec/*.dylib

src/Makefile.in

@@ -0,0 +1,25 @@
+CC = @CC@
+
+CFLAGS = @CFLAGS@
+LOCAL_CFLAGS = @LOCAL_CFLAGS@
+DEFS = @DEFS@
+LOCAL_DEFS = @LOCAL_DEFS@
+
+CCFLAGS = $(DEFS) $(LOCAL_DEFS) $(LOCAL_CFLAGS) $(CFLAGS)
+
+SHOBJ_CC = @SHOBJ_CC@
+SHOBJ_CFLAGS = @SHOBJ_CFLAGS@
+SHOBJ_LD = @SHOBJ_LD@
+SHOBJ_LDFLAGS = @SHOBJ_LDFLAGS@
+SHOBJ_XLDFLAGS = @SHOBJ_XLDFLAGS@
+SHOBJ_LIBS = @SHOBJ_LIBS@
+SHOBJ_STATUS = @SHOBJ_STATUS@
+
+.c.o:
+	$(SHOBJ_CC) $(SHOBJ_CFLAGS) $(CCFLAGS) -c -o $@ $<
+
+../libexec/rbenv-realpath.dylib: realpath.o
+	$(SHOBJ_LD) $(SHOBJ_LDFLAGS) $(SHOBJ_XLDFLAGS) -o $@ realpath.o $(SHOBJ_LIBS)
+
+clean:
+	rm -f *.o ../libexec/*.dylib

src/configure

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+set -e
+
+src_dir="${0%/*}"
+
+CC="${CC:-gcc}"
+
+if ! which "$CC" &>/dev/null; then
+  echo "no compiler found: $CC" >&2
+  exit 1
+fi
+
+case "$(uname -s)" in
+Darwin* )
+  host_os="darwin$(uname -r)"
+  ;;
+OpenBSD* )
+  host_os="openbsd$(uname -r)"
+  ;;
+* )
+  host_os="linux-gnu"
+esac
+
+eval "$("$src_dir"/shobj-conf -C "$CC" -o "$host_os")"
+
+sed "
+  s,@CC@,${CC},
+  s,@CFLAGS@,${CFLAGS},
+  s,@LOCAL_CFLAGS@,${LOCAL_CFLAGS},
+  s,@DEFS@,${DEFS},
+  s,@LOCAL_DEFS@,${LOCAL_DEFS},
+  s,@SHOBJ_CC@,${SHOBJ_CC},
+  s,@SHOBJ_CFLAGS@,${SHOBJ_CFLAGS},
+  s,@SHOBJ_LD@,${SHOBJ_LD},
+  s,@SHOBJ_LDFLAGS@,${SHOBJ_LDFLAGS//,/\,},
+  s,@SHOBJ_XLDFLAGS@,${SHOBJ_XLDFLAGS//,/\,},
+  s,@SHOBJ_LIBS@,${SHOBJ_LIBS},
+  s,@SHOBJ_STATUS@,${SHOBJ_STATUS},
+" "$src_dir"/Makefile.in > "$src_dir"/Makefile

test/run

@@ -2,6 +2,7 @@
 set -e
 
 if [ -n "$RBENV_NATIVE_EXT" ]; then
+  src/configure
   make -C src
 fi