cmake: allow CURL_LTO regardless of CURL_BUILD_TYPE, enable in CI

Before this patch `CURL_LTO` only applied to builds explicitly
configured for `Release` or `RelWithDebInfo` via `CURL_BUILD_TYPE`.

After this patch it enables LTO regardess of build type.

Also:
- GHA/linux: enable LTO in an existing job.

Follow-up to 4ccf7622db #4799
Closes #15829

.github/workflows/linux.yml

@@ -169,10 +169,10 @@ jobs:
             install_packages: zlib1g-dev libkrb5-dev clang
             configure: CC=clang --with-openssl --with-gssapi --enable-debug --disable-docs --disable-manual
 
-          - name: openssl clang krb5
+          - name: openssl clang krb5 LTO
             install_packages: zlib1g-dev libkrb5-dev clang
             install_steps: skipall
-            generate: -DCURL_USE_OPENSSL=ON -DCURL_USE_GSSAPI=ON -DENABLE_DEBUG=ON
+            generate: -DCURL_USE_OPENSSL=ON -DCURL_USE_GSSAPI=ON -DENABLE_DEBUG=ON -DCURL_LTO=ON
 
           - name: openssl !ipv6
             configure: --with-openssl --disable-ipv6 --enable-debug --disable-unity

lib/CMakeLists.txt

@@ -105,9 +105,7 @@ if(SHARE_LIB_OBJECT)
     set_property(TARGET ${LIB_OBJECT} APPEND PROPERTY COMPILE_DEFINITIONS "CURL_HIDDEN_SYMBOLS")
   endif()
   if(CURL_HAS_LTO)
-    set_target_properties(${LIB_OBJECT} PROPERTIES
-      INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE
-      INTERPROCEDURAL_OPTIMIZATION_RELWITHDEBINFO TRUE)
+    set_target_properties(${LIB_OBJECT} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
   endif()
 
   target_include_directories(${LIB_OBJECT} INTERFACE
@@ -138,9 +136,7 @@ if(BUILD_STATIC_LIBS)
     set_property(TARGET ${LIB_STATIC} APPEND PROPERTY COMPILE_DEFINITIONS "CURL_HIDDEN_SYMBOLS")
   endif()
   if(CURL_HAS_LTO)
-    set_target_properties(${LIB_STATIC} PROPERTIES
-      INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE
-      INTERPROCEDURAL_OPTIMIZATION_RELWITHDEBINFO TRUE)
+    set_target_properties(${LIB_STATIC} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
   endif()
 
   target_include_directories(${LIB_STATIC} INTERFACE
@@ -177,9 +173,7 @@ if(BUILD_SHARED_LIBS)
     set_property(TARGET ${LIB_SHARED} APPEND PROPERTY COMPILE_DEFINITIONS "CURL_HIDDEN_SYMBOLS")
   endif()
   if(CURL_HAS_LTO)
-    set_target_properties(${LIB_SHARED} PROPERTIES
-      INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE
-      INTERPROCEDURAL_OPTIMIZATION_RELWITHDEBINFO TRUE)
+    set_target_properties(${LIB_SHARED} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
   endif()
 
   target_include_directories(${LIB_SHARED} INTERFACE

src/CMakeLists.txt

@@ -104,9 +104,7 @@ target_compile_definitions(curltool PUBLIC "UNITTESTS" "CURL_STATICLIB")
 target_link_libraries(curltool PRIVATE ${CURL_LIBS})
 
 if(CURL_HAS_LTO)
-  set_target_properties(${EXE_NAME} PROPERTIES
-    INTERPROCEDURAL_OPTIMIZATION_RELEASE TRUE
-    INTERPROCEDURAL_OPTIMIZATION_RELWITHDEBINFO TRUE)
+  set_target_properties(${EXE_NAME} PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
 endif()
 
 if(ENABLE_UNICODE AND MINGW)