CMake: Default build flavor, HAVE_LIBPNG define

"--flavor"/"-f" flag not specified on the command line: Complain loudly,
print help and exit. The script used to default to "Unix Makefiles".

Add missing "-DHAVE_LIBPNG" compiler command line define when the PNG
library is detected/present for screen capture support.

cmake/cmake-builder.sh

@@ -7,10 +7,6 @@ showHelp()
     cat <<EOF
 Configure and build simh simulators on Linux and *nix-like platforms.
 
-Subdirectories:
-cmake/build-unix:  Makefile-based build simulators
-cmake/build-ninja: Ninja build-based simulators
-
 Options:
 --------
 --clean (-x)      Remove the build subdirectory
@@ -23,7 +19,7 @@ Options:
 --testonly        Do not build, execute the 'ctest' test cases
 --installonly     Do not build, install the SIMH simulators
 
---flavor (-f)     Specifies the build flavor. Valid flavors are:
+--flavor (-f)     [Required] Specifies the build flavor. Valid flavors are:
                     unix
                     ninja
                     xcode
@@ -57,8 +53,8 @@ generateArgs=
 buildArgs=
 buildPostArgs=""
 buildClean=
-buildFlavor="Unix Makefiles"
-buildSubdir=build-unix
+buildFlavor=
+buildSubdir=
 buildConfig=Release
 testArgs=
 notest=no
@@ -276,6 +272,14 @@ while true; do
     esac
 done
 
+# Sanity check: buildSubdir should be set, unless the '-f' flag wasn't present.
+if [ "x${buildSubdir}" = x ]; then
+  echo ""
+  echo "${scriptName}: Build flavor is NOT SET -- see the \"--flavor\"/\"-f\" flag in the help."
+  echo ""
+  showHelp
+fi
+
 ## Determine the SIMH top-level source directory:
 simhTopDir=$(${dirname} $(${realpath} $0))
 while [ "x${simhTopDir}" != x -a ! -f "${simhTopDir}/CMakeLists.txt" ]; do
@@ -296,6 +300,7 @@ if [[ x"$buildClean" != x ]]; then
     echo "${scriptName}: Cleaning ${buildSubdir}"
     rm -rf ${buildSubdir}
 fi
+
 if [[ ! -d ${buildSubdir} ]]; then
     mkdir ${buildSubdir}
 fi

cmake/dep-link.cmake

@@ -128,6 +128,8 @@ IF (WITH_VIDEO)
     ENDIF ()
 
     IF (PNG_FOUND)
+        target_compile_definitions(simh_video INTERFACE HAVE_LIBPNG)
+
         if (TARGET PNG::PNG)
             target_link_libraries(simh_video INTERFACE PNG::PNG)
             list(APPEND VIDEO_PKG_STATUS "interface PNG")