Get rid of odd ENABLE_SYMBOLS=FALSE

* None of the other ENABLE_* support anything but TRUE or empty.

* Other checks for -n "$ENABLE_SYMBOLS" in configure.ac are confused by it.

* solenv/gbuild/gbuild.help.txt talks about "If not empty", not mentioning "FALSE".

* A missing (redundant) explicit --disable-symbols left gb_SYMBOL uninitialized
  (instead of setting it to $(false))---but without consequences, as all the places
  checking gb_SYMBOL explicitly check for $(true).

Change-Id: Id004189de27a7936862dab3a5fd84d549c06af2b
Reviewed-on: https://gerrit.libreoffice.org/27057Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -4044,11 +4044,7 @@ if test -n "$enable_symbols" -a "$enable_symbols" != "no"; then
     fi
     AC_MSG_RESULT([yes])
 else
-    if test -n "$enable_symbols" -a "$enable_symbols" = "no"; then
-        ENABLE_SYMBOLS="FALSE"
-    else
-        ENABLE_SYMBOLS=
-    fi
+    ENABLE_SYMBOLS=
     AC_MSG_RESULT([no])
 fi
 AC_SUBST(ENABLE_SYMBOLS)

solenv/gbuild/gbuild.mk

@@ -117,12 +117,10 @@ gb_Deliver_HARDLINK := $(true)
 endif
 
 # note: ENABLE_CRASHDUMP turns on gb_SYMBOL
-ifeq ($(or $(ENABLE_SYMBOLS),$(enable_symbols)),FALSE)
-gb_SYMBOL := $(false)
-else
 ifneq ($(strip $(ENABLE_SYMBOLS)$(enable_symbols)$(ENABLE_CRASHDUMP)),)
 gb_SYMBOL := $(true)
-endif
+else
+gb_SYMBOL := $(false)
 endif
 
 ifneq ($(strip $(ENABLE_PCH)),)