Allow overriding target compiler and linker with qmake's one

Using make_global_settings, we can pass along the compiler and linker
that qmake uses, still allowing overriding it through environment variables.
We generate qmake_extras.gypi and populated with the host and target compilers
so that it also works when cross compiling.

Based on patch by Pierre Rossi <pierre.rossi@digia.com>

Change-Id: Ia3789abeea88f4d3cde75c2cf07cc9de66d7afbf
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>

build/gyp_qtwebengine

@@ -65,6 +65,9 @@ def additional_include_files(args=[]):
   # Used for additional build tweaks such as file exclusions
   AddInclude(os.path.join(qtwebengine_src, 'build', 'qtwebengine_extras.gypi'))
 
+  # Common stuff we generate and extract from qmake
+  AddInclude(os.path.join(qtwebengine_src, 'build', 'qmake_extras.gypi'))
+
   # Optionally add supplemental .gypi files if present.
   supplements = glob.glob(os.path.join(chrome_src, '*', 'supplement.gypi'))
   for supplement in supplements:

build/qmake_extras/host/host.pro

+# Prevent generating a makefile that attempts to create a lib
+TEMPLATE = aux
+
+# Pick up the host toolchain
+option(host_build)
+
+GYPI_CONTENTS = "{" \
+                "  'make_global_settings': [" \
+                "    ['CC.host', '$$which($$QMAKE_CC)']," \
+                "    ['CXX.host', '$$which($$QMAKE_CXX)']," \
+                "    ['LD.host', '$$which($$QMAKE_LINK)'],"
+
+GYPI_FILE = $$absolute_path('build/qmake_extras.gypi', $$QTWEBENGINE_ROOT)
+write_file($$GYPI_FILE, GYPI_CONTENTS)

build/qmake_extras/qmake_extras.pro

+TEMPLATE = subdirs
+
+CONFIG += ordered
+
+SUBDIRS = host \
+          target

build/qmake_extras/target/target.pro

+# Prevent generating a makefile that attempts to create a lib
+TEMPLATE = aux
+
+GYPI_CONTENTS += "    ['CC', '$$which($$QMAKE_CC)']," \
+                 "    ['CXX', '$$which($$QMAKE_CXX)']," \
+                 "    ['LD', '$$which($$QMAKE_LINK)'],"
+GYPI_CONTENTS += "  ],"
+GYPI_CONTENTS += "}"
+
+GYPI_FILE = $$absolute_path('build/qmake_extras.gypi', $$QTWEBENGINE_ROOT)
+
+!exists($$GYPI_FILE): error("-- $$GYPI not found --")
+
+# Append to the file already containing the host settings.
+write_file($$GYPI_FILE, GYPI_CONTENTS, append)

qtwebengine.pro

@@ -4,7 +4,8 @@ CONFIG += ordered
 
 # The first three subdirs contain dummy .pro files that are used by qmake
 # to generate a corresponding .gyp file
-SUBDIRS = resources \
+SUBDIRS = build/qmake_extras \ # Phony pro file that extracts things like compiler and linker from qmake
+          resources \
           shared \
           lib \
           process \