$ diff -Naur ./CMake/install.cmake.orig ./CMake/install.cmake
--- ./CMake/install.cmake.orig	2021-07-23 19:24:01.342947700 +0200
+++ ./CMake/install.cmake	2021-07-23 19:26:52.966549200 +0200
@@ -113,7 +113,7 @@
   DESTINATION ${FLTK_BINDIR}
 )
 
-if (UNIX OR MSYS OR (MINGW AND CMAKE_CROSSCOMPILING))
+if (UNIX OR MINGW)
   macro(INSTALL_MAN FILE LEVEL)
     install(FILES
       ${CMAKE_CURRENT_SOURCE_DIR}/documentation/src/${FILE}.man
@@ -129,4 +129,4 @@
   INSTALL_MAN (checkers 6)
   INSTALL_MAN (sudoku 6)
 
-endif (UNIX OR MSYS OR (MINGW AND CMAKE_CROSSCOMPILING))
+endif (UNIX OR MINGW)

$ diff -Naur ./CMake/setup.cmake.orig ./CMake/setup.cmake
--- ./CMake/setup.cmake.orig	2021-07-23 19:24:01.365947900 +0200
+++ ./CMake/setup.cmake	2021-07-23 19:28:56.438734100 +0200
@@ -57,13 +57,13 @@
 #######################################################################
 
 # set where config files go
-if (WIN32 AND NOT CYGWIN)
+if (WIN32 AND NOT CYGWIN AND NOT MINGW)
   set (FLTK_CONFIG_PATH CMake)
 elseif (APPLE)
   set (FLTK_CONFIG_PATH FLTK.framework/Resources/CMake)
 else ()
   set (FLTK_CONFIG_PATH ${FLTK_DATADIR}/fltk)
-endif (WIN32 AND NOT CYGWIN)
+endif (WIN32 AND NOT CYGWIN AND NOT MINGW)
 
 include(TestBigEndian)
 TEST_BIG_ENDIAN(WORDS_BIGENDIAN)
