depends: Specify CMake generator explicitly

Building packages in depends implies using GNU Make. However, this
assumption can be wrong in environments where the `CMAKE_GENERATOR`
variable is set.
This change explicitly makes CMake use the "Unix Makefiles" generator.

depends/funcs.mk

@@ -179,7 +179,8 @@ $(1)_cmake=env CC="$$($(1)_cc)" \
                CXX="$$($(1)_cxx)" \
                CXXFLAGS="$$($(1)_cppflags) $$($(1)_cxxflags)" \
                LDFLAGS="$$($(1)_ldflags)" \
-               cmake -DCMAKE_INSTALL_PREFIX:PATH="$$($($(1)_type)_prefix)" \
+               cmake -G "Unix Makefiles" \
+               -DCMAKE_INSTALL_PREFIX:PATH="$$($($(1)_type)_prefix)" \
                -DCMAKE_AR=`which $$($(1)_ar)` \
                -DCMAKE_NM=`which $$($(1)_nm)` \
                -DCMAKE_RANLIB=`which $$($(1)_ranlib)` \