[PATCH v2] build: avoid possible confusion between GNU/XPG4 make on Solaris

On a Solaris 10 system with Solaris XPG4 make installed as /usr/xpg4/bin/make,
GNU make installed as /usr/local/bin/make, and with /usr/local/bin appearing
in $PATH *before* /usr/xpg4/bin, I was seeing errors like this upon invoking
"make all":

        SUBDIR perl
    make: Warning: Ignoring DistributedMake -o option
    Usage : make [ -f makefile ][ -K statefile ]...
    make: Fatal error: Unknown option `-C'

This happens because the Git's Makefiles, when running on Solaris, sanitize
$PATH by prepending /usr/xpg6/bin and /usr/xpg4/bin to it, but in the setup
described above such a behaviour has the unintended consequence of forcing
the use of Solaris make in recursive make invocations, even if the $(MAKE)
macro is being correctly used in them; this happens because, in that setup,
the original GNU make process was invoked simply as "make".

To avoid such an issue, we instruct our Makefile to redefine $(MAKE) to
point to the absolute path of the originally-invoked make program.

Helped-by: Thomas Rast [off-list ref]
Signed-off-by: Stefano Lattarini <redacted>
---
 Makefile |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/Makefile b/Makefile
index e4f8e0e..e71d688 100644
--- a/Makefile
+++ b/Makefile

@@ -303,6 +303,21 @@ ifdef MSVC
 	uname_O := Windows
 endif
 
+# This Makefile will possibly sanitize PATH by prepending system-specific
+# directories to it (e.g., /usr/xpg4/bin on Solaris).  This can become
+# problematic for recursive make invocations, if one of those directories
+# contains a "make" program and the user has called GNU make by simply
+# invoking "make" (this can happen e.g. when GNU make has been installed
+# as /usr/local/bin/make).  To avoid such issues, we redefine $(MAKE) to
+# point to the absolute path of the originally-invoked make program.
+original_MAKE := $(MAKE)
+MAKE := $(shell command -v $(firstword $(original_MAKE)) 2>/dev/null)
+ifeq ($(MAKE),)
+	MAKE := $(original_MAKE)
+else
+	MAKE += $(wordlist 2,$(words $(original_MAKE)),$(original_MAKE))
+endif
+
 # CFLAGS and LDFLAGS are for the users to override from the command line.
 
 CFLAGS = -g -O2 -Wall

-- 
1.7.9