cmake-env.mk: Allow using BUILD_SHARED_LIBS and fix legacy behavior (#6784)

Author: th0ma7

mk/spksrc.cross-cmake-env.mk

@@ -38,7 +38,7 @@ CMAKE_INSTALL_RPATH_USE_LINK_PATH = TRUE
 CMAKE_BUILD_WITH_INSTALL_RPATH = TRUE
 
 # Allow building shared libraries to be manually set
-ifeq ($(filter -DBUILD_SHARED_LIBS%,$(CMAKE_ARGS)),)
+ifeq ($(or $(filter -DBUILD_SHARED_LIBS%,$(CMAKE_ARGS)),$(strip $(BUILD_SHARED_LIBS))),)
 BUILD_SHARED_LIBS = ON
 endif
 
@@ -61,8 +61,10 @@ CMAKE_ARGS += -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=$(CMAKE_FIND_ROOT_PATH_MODE_PR
 CMAKE_ARGS += -DCMAKE_INSTALL_RPATH=$(CMAKE_INSTALL_RPATH)
 CMAKE_ARGS += -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=$(CMAKE_INSTALL_RPATH_USE_LINK_PATH)
 CMAKE_ARGS += -DCMAKE_BUILD_WITH_INSTALL_RPATH=$(CMAKE_BUILD_WITH_INSTALL_RPATH)
+ifneq ($(strip $(BUILD_SHARED_LIBS)),)
 CMAKE_ARGS += -DBUILD_SHARED_LIBS=$(BUILD_SHARED_LIBS)
 endif
+endif
 
 # Use native cmake (latest stable)
 ifeq ($(strip $(USE_NATIVE_CMAKE)),1)