depends: Prepend CPPFLAGS to C{,XX}FLAGS for CMake

This is similar to how we do it for qt.mk.

(cherry picked from commit bitcoin/bitcoin@b3f541f618)

depends/funcs.mk

@@ -174,9 +174,9 @@ $(1)_autoconf += LDFLAGS="$$($(1)_ldflags)"
 endif
 
 $(1)_cmake=env CC="$$($(1)_cc)" \
-               CFLAGS="$$($(1)_cflags)" \
+               CFLAGS="$$($(1)_cppflags) $$($(1)_cflags)" \
                CXX="$$($(1)_cxx)" \
-               CXXFLAGS="$$($(1)_cxxflags)" \
+               CXXFLAGS="$$($(1)_cppflags) $$($(1)_cxxflags)" \
              cmake -DCMAKE_INSTALL_PREFIX:PATH="$$($($(1)_type)_prefix)"
 ifneq ($($(1)_type),build)
 ifneq ($(host),$(build))