build-standalone fails with "configP.h: file not found"

[shirok]

t.scm:

(define (main args) (print "Hello, world.") 0)

and:

$ gosh tools/build-standalone t.scm
gcc -fPIC  -I/usr/lib/gauche-0.98/0.9.14-p1/include -o t tAKoF4N.c -L/usr/lib/gauche-0.98/0.9.14-p1/x86_64-pc-linux-gnu -lgauche-static-0.98   -lgdbm   -lz  -lmbedtls -lmbedx509 -lmbedcrypto   -lmbedtls -lcrypt -lrt -lm  -lpthread
In file included from tAKoF4N.c:3:
/usr/lib/gauche-0.98/0.9.14-p1/include/gauche.h:45:10: fatal error: gauche/priv/configP.h: No such file or directory
   45 | #include "gauche/priv/configP.h" /* Only used while compiling Gauche itself */
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

configP.h is included only if LIBGAUCHE_BODY is set before gauche.h.
build-standalone shouldn't set it, or we need a different flag to control configP.h inclusion.

Additionally, tests failed to detect this because from the source directiory confiP.h is visible.

[shirok]

Aah, gauche/static.h turns LIBGAUCHE_BODY on.

[shirok]

Fixed.
There's a test of build-standalone in test/script.scm but it failed to detect this case, since in the testing environment configP.h is visible. It'll be a bit tricky to construct tests that detects this, for we need a temporary installment. We could use github workflow, maybe.