complibs: introduce a two-stage select

Directly select-ing the required companion libraries means we can not
disable them. That's OK for now, as we systematically build them when
they are required.

But with distros coming up-to-speed, we will need to disable the build
later-on.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

config/cc/gcc.in

@@ -161,24 +161,24 @@ config CC_GCC_HAS_LTO
 config CC_GCC_USE_GMP_MPFR
     bool
     default n
-    select GMP
-    select MPFR
+    select GMP_NEEDED
+    select MPFR_NEEDED
 
 config CC_GCC_USE_PPL_CLOOG
     bool
     default n
-    select PPL
-    select CLOOG
+    select PPL_NEEDED
+    select CLOOG_NEEDED
 
 config CC_GCC_USE_MPC
     bool
     default n
-    select MPC
+    select MPC_NEEDED
 
 config CC_GCC_USE_LIBELF
     bool
     default n
-    select LIBELF
+    select LIBELF_NEEDED
 
 config CC_VERSION
     string

config/companion_libs.in

@@ -4,6 +4,46 @@
 
 menu "Companion libraries"
 
+config COMPLIBS_NEEDED
+    bool
+    default n
+
+config GMP_NEEDED
+    bool
+    default n
+    select GMP
+    select COMPLIBS_NEEDED
+
+config MPFR_NEEDED
+    bool
+    default n
+    select MPFR
+    select COMPLIBS_NEEDED
+
+config PPL_NEEDED
+    bool
+    default n
+    select PPL
+    select COMPLIBS_NEEDED
+
+config CLOOG_NEEDED
+    bool
+    default n
+    select CLOOG
+    select COMPLIBS_NEEDED
+
+config MPC_NEEDED
+    bool
+    default n
+    select MPC
+    select COMPLIBS_NEEDED
+
+config LIBELF_NEEDED
+    bool
+    default n
+    select LIBELF
+    select COMPLIBS_NEEDED
+
 config COMPLIBS
     bool
     default n