do not bother trying to keep PCH and non-PCH dependencies correct together

When compiling using a PCH, includes included using the PCH usually don't
show up in compiler dependencies output, so the only the PCH has a dependency
on the include, and the source has only a transitive dependency. Switching
to non-PCH (e.g. ENABLE_PCH= on cmdline) means the dependency is lost.
Trying with the PCH timestamp was a nice try, but it can sometimes still
miss something. So whoever wants to switch needs cleaning first or
just know what they're doing.

solenv/gbuild/LinkTarget.mk

@@ -877,7 +877,9 @@ $(call gb_CxxObject_get_target,$(2)) : | $(call gb_LinkTarget_get_headers_target
 $(call gb_CxxObject_get_target,$(2)) : T_CXXFLAGS += $(3)
 $(call gb_CxxObject_get_target,$(2)) : \
 	OBJECTOWNER := $(call gb_Object__owner,$(2),$(1))
+ifeq ($(gb_ENABLE_PCH),$(true))
 $(call gb_CxxObject_get_target,$(2)) : $(call gb_PrecompiledHeader_get_timestamp,$(1))
+endif
 
 ifeq ($(gb_FULLDEPS),$(true))
 $(call gb_LinkTarget_get_dep_target,$(1)) : CXXOBJECTS += $(2)
@@ -974,8 +976,9 @@ $(call gb_GenCxxObject_get_target,$(2)) : T_CXXFLAGS += $(3)
 $(call gb_GenCxxObject_get_target,$(2)) : \
 	OBJECTOWNER := $(call gb_Object__owner,$(2),$(1))
 $(call gb_GenCxxObject_get_target,$(2)) : GEN_CXX_SOURCE := $(call gb_GenCxxObject_get_source,$(2),$(1))
-
+ifeq ($(gb_ENABLE_PCH),$(true))
 $(call gb_GenCxxObject_get_target,$(2)) : $(call gb_PrecompiledHeader_get_timestamp,$(1))
+endif
 
 ifeq ($(gb_FULLDEPS),$(true))
 $(call gb_LinkTarget_get_dep_target,$(1)) : GENCXXOBJECTS += $(2)
@@ -1020,9 +1023,11 @@ endef
 
 define gb_LinkTarget_add_noexception_object
 $(call gb_LinkTarget_add_cxxobject,$(1),$(2),$(gb_LinkTarget_NOEXCEPTIONFLAGS) $(call gb_LinkTarget__get_cxxflags,$(3)))
+ifeq ($(gb_ENABLE_PCH),$(true))
 # noexception objects are rare, so generate matching .pch only when needed
 $(call gb_CxxObject_get_target,$(2)) : $(call gb_NoexPrecompiledHeader_get_timestamp,$(1))
 $(call gb_LinkTarget_get_target,$(1)) : PCHOBJS = $$(PCHOBJEX) $$(PCHOBJNOEX)
+endif
 endef
 
 define gb_LinkTarget_add_exception_object

solenv/gbuild/gbuild.help.txt

@@ -69,7 +69,6 @@ INTERACTIVE VARIABLES:
                        3... = degugging information + no optimizations + extra
                               debug output (usually extremely verbose). Levels
                               > 2 are not used very much.
-       ENABLE_PCH      If not empty, use precompiled headers.
        TIMELOG / timelog
                        If not empty enable the RTL_LOGFILE_* time logging facility.
                        export RTL_LOGFILE=rtl_logfile.nopid when running office, to