Add build steps to check the committed files against "make clean".

If "make clean" on a new checkout causes diffs, you accidentally committed
one of the build products currently listed in "make clean", and you
should `git rm` it again.

If "make clean" on a dirty/built checkout causes diffs, then either
(1) the "make" process modified some tracked file which is not removed
by "make clean", i.e. either (1A) it is a build product which needs to
be `git rm`ed AND added under "make clean", or (1B) the build process
is modifying some source file, which is highly weird and shouldn't happen;
or else (2) "make" created some untracked file which is not removed
by "make clean", i.e., you should add that build product to "make clean".

The Travis build that uses "mymake" now does the same "mv" trick as Github CI,
to avoid having the executable sometimes named "./hyperrogue" and sometimes
named "./hyper". Why are these filenames different, anyway?

.travis.yml

@@ -177,6 +177,12 @@ before_install:
   fi
 
 script:
+- |-
+  make -f Makefile.simple clean
+  if [[ "$(git status --porcelain)" ]]; then
+    git status
+    exit 'A build artifact was committed; git rm it and try again'
+  fi
 - |-
   # Build hyperrogue.
   if [[ "$TRAVIS_BUILD_SYSTEM" == "autotools" ]]; then
@@ -192,6 +198,7 @@ script:
     else
       ./mymake
     fi
+    mv hyper hyperrogue
   elif [[ "$TRAVIS_BUILD_SYSTEM" == "emscripten" ]]; then
     docker run --rm -v $(pwd):/src trzeci/emscripten make -f Makefile.simple emscripten
   else
@@ -201,11 +208,19 @@ script:
   # Test hyperrogue.
   if [[ "$TRAVIS_BUILD_SYSTEM" == "emscripten" ]]; then
     ls -lAF hyper.html hyper.js hyper.wasm
-  elif [[ "$TRAVIS_BUILD_SYSTEM" == "mymake" ]]; then
-    ./hyper --help
   else
     ./hyperrogue --help
   fi
+- |-
+  if [[ "$TRAVIS_BUILD_SYSTEM" == "autotools" ]]; then
+    make clean
+  else
+    make -f Makefile.simple clean
+  fi
+  if [[ "$(git status --porcelain)" ]]; then
+    git status
+    exit 'make clean did not return the repository to its pre-build state'
+  fi
 - |-
   # Test "make dist". ("make distcheck" is expected to fail.)
   if [[ "$TRAVIS_BUILD_SYSTEM" == "autotools" ]]; then make dist; fi

Makefile.am

@@ -21,6 +21,7 @@ langen_SOURCES = langen.cpp
 langen_CXXFLAGS = -O0 -std=c++11 ${AM_CXXFLAGS}
 makeh_SOURCES = makeh.cpp
 BUILT_SOURCES = language-data.cpp autohdr.h
+CLEANFILES = language-data.cpp autohdr.h
 
 language-data.cpp: langen
 	./langen > language-data.cpp