Disallow in-source builds

Rationale:

1. Building in-source creates a lot of untracked files in the source
   tree. We need to take care not to commit any of them, hence we
   have to maintain .gitignore files in several directories.

2. Tests reveal that there are even files in git that are modified
   in the source tree when building docs etc. which means e.g.
   that builds can't be "repeated" after upgrading the sources, and
   that those files could be committed by accident. To avoid this
   we'd have to reorganize the build procedure and make it even more
   complex rather than simplifying it.

3. With autoconf/configure/make we didn't have a choice, i.e. we had
   to allow in-source builds with configure/Makefiles anyway. This
   is no longer true since we removed configure/make support.

Note, todo: if this commit doesn't introduce unexpected issues, then
   we can remove a lot of stuff from the .gitignore files, but this
   is left for a later commit.

1 files changed, 17 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 420e379c1..08548b2b4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -16,6 +16,23 @@
 #
 
 #######################################################################
+# Prevent building in the source tree - don't pollute the sources
+#######################################################################
+
+if("${CMAKE_BINARY_DIR}" STREQUAL "${CMAKE_SOURCE_DIR}")
+  message("")
+  message(STATUS "=======================================================================")
+  message(STATUS " NOTE: In-source builds of FLTK are disabled to prevent writing to the")
+  message(STATUS " NOTE: source tree. Please create a subfolder (e.g. 'build') and use")
+  message(STATUS " NOTE: `cmake ..` inside it or use for instance `cmake . -B build`.")
+  message(STATUS " NOTE: CMake will, however, create CMakeCache.txt and CMakeFiles/* .")
+  message(STATUS " NOTE: You must delete CMakeCache.txt and CMakeFiles/* manually.")
+  message(STATUS "=======================================================================")
+  message("")
+  message(FATAL_ERROR "*** In-source builds are disabled, please read notes above! ***")
+endif()
+
+#######################################################################
 # Set CMake minimum version first: must be set before `project()`
 #######################################################################