package/meson: fix shared build issue due to --static flag always passed to pkg-config

package/meson: fix shared build issue due to --static flag always passed to pkg-config

Since cf75d7da98596580eee5a9b5e1a3e156ad832099 we have a build failures when
building libgbm.so when valgrind package is selected because --static is always
passed to pkg-config even for shared build.

Even if -Dvalgrind=false on meson command line to build mesa, the valgrind
libraries come from pkg-config libdrm...

output/host/bin/pkg-config libdrm --libs --static
-L[...]/sysroot/usr/lib -ldrm -lm -L[...]/sysroot/usr/lib/valgrind
-lcoregrind-arm64-linux -lvex-arm64-linux -lgcc

... and break the build.

See initial discussions:
http://lists.busybox.net/pipermail/buildroot/2020-June/284543.html

This is due to a wrong condition test added by the patch
0004-mesonbuild-dependencies-base.py-add-pkg_config_stati.patch.

Indeed, pkg_config_static is a string, not a boolean; it is set to
either 'true' or 'aflse' by our meson package infra. Since the returned
object is a string, do not pass a boolean, but pas None (we only want to
test against the 'true' string, so we don't care what we get back when
it is not set, which never happens in Buildroot).

Before this patch, the issue can be reproduced using the following defconfig:
    BR2_aarch64=y
    BR2_TOOLCHAIN_EXTERNAL=y
    BR2_PACKAGE_VALGRIND=y
    BR2_PACKAGE_MESA3D=y
    BR2_PACKAGE_MESA3D_DRI_DRIVER_SWRAST=y

Fixes:
http://autobuild.buildroot.net/results/1b5/1b58d73ecbbe1af2c3e140563d696cf32d1c4a5a/build-end.log

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: James Hilliard <james.hilliard1@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: slightly reword the commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>