cmake: allow disabling ENABLE_CURLDEBUG with ENABLE_DEBUG=ON

Before this patch, `ENABLE_CURLDEBUG` (memory tracking) was
unconditionally enabled when `ENABLE_DEBUGBUILD` was set. This made
testing some build configurations complicated. To fix this, after this
patch we only enable `ENABLE_CURLDEBUG` if not already defined by the
user.

This allows to use this config:
ENABLE_DEBUGBUILD=ON
ENABLE_CURLDEBUG=OFF
to enable debug features, without also enabling memory tracking.

This is important because some other build methods allow to set
one of these features but not the other. This patch allows to test
any combination with CMake.

This makes it unnecessary to use the workaround of passing
`-DDEBUGBUILD` via `CMAKE_C_FLAGS`. Which has the disadvantage
that our CMake logic cannot easily detect it, e.g. for disabling
symbol hiding on Windows for `ENABLE_DEBUG`/`DEBUGBUILD` builds.

Cherry-picked from curl#13718
Closes #xxxxx

CMakeLists.txt

@@ -121,7 +121,9 @@ include(PickyWarnings)
 if(ENABLE_DEBUG)
   # DEBUGBUILD will be defined only for Debug builds
   set_property(DIRECTORY APPEND PROPERTY COMPILE_DEFINITIONS $<$<CONFIG:Debug>:DEBUGBUILD>)
-  set(ENABLE_CURLDEBUG ON)
+  if(NOT DEFINED ENABLE_CURLDEBUG)
+    set(ENABLE_CURLDEBUG ON)
+  endif()
 endif()
 
 if(ENABLE_CURLDEBUG)

appveyor.sh

@@ -43,7 +43,7 @@ if [ "${BUILD_SYSTEM}" = 'CMake' ]; then
   [ "${PRJ_CFG}" = 'Release' ] && options+=' -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE='
   [[ "${PRJ_GEN}" = *'Visual Studio'* ]] && options+=' -DCMAKE_VS_GLOBALS=TrackFileAccess=false'
   if [ "${PRJ_GEN}" = 'Visual Studio 9 2008' ]; then
-    [ "${PRJ_CFG}" = 'Debug' ] && [ "${DEBUG}" = 'ON' ] && [ "${SHARED}" = 'ON' ] && SKIP_RUN='Crash on startup in -DDEBUGBUILD shared builds'
+    [ "${PRJ_CFG}" = 'Debug' ] && [ "${DEBUG}" = 'ON' ] && [ "${SHARED}" = 'ON' ] && SKIP_RUN='Crash on startup in ENABLE_DEBUG=ON shared builds'
     # Fails to run without this due to missing MSVCR90.dll / MSVCR90D.dll
     options+=' -DCURL_STATIC_CRT=ON'
   fi