[PATCH libinput] meson.build: enable CPP include check only in case CPP compiler is available

[Peter Seiderer]

Drop hard meson C++/CPP dependency, only needed for the build-time
header inclusion test, build the test only in case C++/CPP compiler
is available.

Signed-off-by: Peter Seiderer <ps.report at gmx.net>
---
Fixes the following buildroot autobuild detected failure([1]):

  The Meson build system
  Version: 0.49.0
  Source dir: .../build/libinput-1.12.5
  Build dir: .../build/libinput-1.12.5/build
  Build type: cross build
  Project name: libinput
  Project version: 1.12.5
  Native C compiler: cc (gcc 4.8.4 "cc (Ubuntu 4.8.4-2ubuntu1~14.04.4) 4.8.4")
  Cross C compiler: .../host/bin/riscv64-buildroot-linux-gnu-gcc (gcc 7.4.0)

  meson.build:1:0: ERROR:  Unknown compiler(s): [['.../host/bin/riscv64-buildroot-linux-gnu-g++']]
  The follow exceptions were encountered:
  Running ".../host/bin/riscv64-buildroot-linux-gnu-g++ --version" gave "[Errno 2] No such file or directory: '.../host/bin/riscv64-buildroot-linux-gnu-g++': '.../host/bin/riscv64-buildroot-linux-gnu-g++'"

[1] http://autobuild.buildroot.net/results/bf4d3d360f635c3524a52b84a72d558770596ed0
---
 meson.build | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/meson.build b/meson.build
index a9c7c185..8183822e 100644
--- a/meson.build
+++ b/meson.build
@@ -1,4 +1,4 @@
-project('libinput', 'c', 'cpp',
+project('libinput', 'c',
 	version : '1.12.5',
 	license : 'MIT/Expat',
 	default_options : [ 'c_std=gnu99', 'warning_level=2' ],
@@ -641,12 +641,14 @@ executable('test-build-linker',
 	   include_directories : [includes_src, includes_include],
 	   dependencies : [ dep_libinput, dep_libinput_util ],
 	   install : false)
-# test including from C++
-executable('test-build-cxx',
-	   'test/build-cxx.cc',
-	   dependencies : [dep_udev],
-	   include_directories : [includes_src, includes_include],
-	   install : false)
+# test including from C++ (in case CPP compiler is available)
+if add_languages('cpp', required: false)
+	executable('test-build-cxx',
+		   'test/build-cxx.cc',
+		   dependencies : [dep_udev],
+		   include_directories : [includes_src, includes_include],
+		   install : false)
+endif
 
 # This is the test suite runner, we allow disabling that one because of
 # dependencies
-- 
2.20.1