package/pkg-meson.mk: handle possibly non existing compilers

To avoid populating the cross-file with non existing compilers,
we tie them to /bin/false.

As explained by @eliebrokeit on the meson IRC channel:

    If you don't have one [a compiler] defined, cross builds won't try
    to autodetect one, IIRC...
    ... however setting it to /bin/false will result in it detecting
    that that doesn't work
    I guess the safest bet is to deliberately poison it with /bin/false

So there we go.

Signed-off-by: Guillaume W. Bres <guillaume.bressaix@gmail.com>
[yann.morin.1998@free.fr: expand with IRC snippet]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/pkg-meson.mk

@@ -68,15 +68,29 @@ else
 PKG_MESON_TARGET_CPU_FAMILY = $(ARCH)
 endif
 
+# To avoid populating the cross-file with non existing compilers,
+# we tie them to /bin/false
+ifeq ($(BR2_INSTALL_LIBSTDCPP),y)
+PKG_MESON_TARGET_CXX = $(TARGET_CXX)
+else
+PKG_MESON_TARGET_CXX = /bin/false
+endif
+
+ifeq ($(BR2_TOOLCHAIN_HAS_FORTRAN),y)
+PKG_MESON_TARGET_FC = $(TARGET_FC)
+else
+PKG_MESON_TARGET_FC = /bin/false
+endif
+
 # Generates sed patterns for patching the cross-compilation.conf template,
 # since Flags might contain commas the arguments are passed indirectly by
 # variable name (stripped to deal with whitespaces).
 # Arguments are variable containing cflags, cxxflags, ldflags, fcflags
 define PKG_MESON_CROSSCONFIG_SED
         -e "s%@TARGET_CC@%$(TARGET_CC)%g" \
-        -e "s%@TARGET_CXX@%$(TARGET_CXX)%g" \
+        -e "s%@TARGET_CXX@%$(PKG_MESON_TARGET_CXX)%g" \
         -e "s%@TARGET_AR@%$(TARGET_AR)%g" \
-        -e "s%@TARGET_FC@%$(TARGET_FC)%g" \
+        -e "s%@TARGET_FC@%$(PKG_MESON_TARGET_FC)%g" \
         -e "s%@TARGET_STRIP@%$(TARGET_STRIP)%g" \
         -e "s%@TARGET_ARCH@%$(PKG_MESON_TARGET_CPU_FAMILY)%g" \
         -e "s%@TARGET_CPU@%$(GCC_TARGET_CPU)%g" \